Coordination of dissolved transition metals in pristine battery electrolyte solutions determined by NMR and EPR spectroscopy

Abstract

The solvation of dissolved transition metal ions in lithium-ion battery electrolytes is not well-characterised experimentally, although it is important for battery degradation mechanisms governed by metal dissolution, deposition, and reactivity in solution. This work identifies the coordinating species in the Mn²⁺ and Ni²⁺ solvation spheres in LiPF₆/LiTFSI–carbonate electrolyte solutions by examining the electron–nuclear spin interactions, which are probed by pulsed EPR and paramagnetic NMR spectroscopy. These techniques investigate solvation in frozen electrolytes and in the liquid state at ambient temperature, respectively, also probing the bound states and dynamics of the complexes involving the ions. Mn²⁺ and Ni²⁺ are shown to primarily coordinate to ethylene carbonate (EC) in the first coordination sphere, while PF₆⁻ is found primarily in the second coordination sphere, although a degree of contact ion pairing does appear to occur, particularly in electrolytes with low EC concentrations. NMR results suggest that Mn²⁺ coordinates more strongly to PF₆⁻ than to TFSI⁻, while the opposite is true for Ni²⁺. This work provides a framework to experimentally determine the coordination spheres of paramagnetic metals in battery electrolyte solutions.

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Introduction

As lithium-ion cells degrade, transition metal ions may dissolve from cathode materials and be deposited at the anode, causing issues including degradation of the solid electrolyte interphase (SEI) at the anode, further SEI formation and increased trapping of active lithium, impedance rise, and capacity loss.^1–5 While many studies have been conducted on the topic of Li⁺ solvation in battery electrolytes, there is far less work surrounding the solvation of dissolved transition metals. Yet an understanding of the nature and strength of coordination of the transition metal ions by the different species in both pristine (fresh) and degraded electrolyte solutions is important to understand and control both the dissolution of transition metals from the cathode and their deposition on the anode, ultimately providing chemical insights into cell degradation pathways.

The picture of Li⁺ solvation in battery electrolyte solutions is itself complicated; however, it is understood that Li⁺ tends to be tetrahedrally solvated^6–10 and that coordination to ethylene carbonate (EC) is preferred over coordination to linear carbonates,^7,9–17 with the extent of ion pairing between Li⁺ and PF₆⁻ being dependent on the salt concentration.^11,18,19 For transition metal solvation, a computational study of Mn²⁺, Ni²⁺, and Co²⁺ solvation by EC showed that the structures would likely be six-coordinate, and that Mn²⁺ desolvation occurs more readily than Ni²⁺ or Co²⁺ desolvation, potentially influencing the role of Mn²⁺ in the SEI.²⁰ Later simulations of the Mn²⁺ solvation shell showed that Mn²⁺ interaction energies follow the order EC > PF₆⁻ > linear carbonates.²¹ Our previous NMR studies have suggested that dissolved transition metals may favour coordination to EC over ethyl methyl carbonate^22,23 and may coordinate preferentially to PF₆⁻ degradation products over pristine electrolyte components.^23,24

In addition to understanding the solvation of dissolved transition metal ions, we aim to understand the solvation of model transition metal salts, and identify whether these model species are truly representative. Most studies aiming to mimic the effects of dissolved metals from cathode materials, including this work, use commercially available M(TFSI)₂ salts.^21,25–34 Although it is assumed that the metal cations dissociate from the TFSI⁻ anions, (i.e., tight M²⁺–TFSI⁻ ion pairs are not formed) due to the high solubility of the metal salts, and their small concentrations within the electrolyte, the extent of coordination between dissolved metals and TFSI⁻ is not yet clear. At these low concentrations, the TFSI⁻ anion is not thought to alter the reduction potential of dissolved transition metals or to affect cell cycling.²⁸ It has also been suggested that because electrolyte solutions containing LiTFSI and LiPF₆ behave similarly with respect to gassing at the negative electrode,^35–37 the TFSI⁻ counterion that is added with transition metals to an LiPF₆ solution should not affect the electrolyte decomposition reactions induced by those transition metals.²⁶ However, a comparison of electrolyte solutions containing added Mn(TFSI)₂ and Mn(PF₆)₂ produced different cycling behaviour in LiFePO₄/graphite cells, even though cells were not negatively affected by the addition of LiTFSI.²⁹ Specifically, differences were observed in the amount of Mn deposited at both electrode surfaces and in the rate of capacity loss, with rapid initial capacity loss caused by Mn(TFSI)₂ and smaller, continual capacity loss with more ongoing parasitic side reactions caused by Mn(PF₆)₂.²⁹ A potential explanation for this is that Mn(TFSI)₂ may not dissociate completely in a typical electrolyte solution comprising LiPF₆, EC, and linear carbonates; such a notion is consistent with calculations showing that the Mn²⁺ interaction energy with TFSI⁻ is even stronger than with EC, PF₆⁻, or linear carbonates.²¹ In a similar vein, a study of Co deposition using the additive Co(NO₃)₂ found deposition of Li₃N, which was not present when LiNO₃ was added to the electrolyte solution.³⁸ We note that the SEIs formed in electrolytes comprising LiTFSI are very different from those formed with LiPF₆, which may also play an indirect role in some of the observed experimental differences. However, if the compounds that are used to mimic the effects of transition metal dissolution are found to alter the transition metal coordination shells, such that the counterions affect the action of dissolved transition metals, then studies using transition metal salts may be non-representative and lead to flawed conclusions about the role of dissolved transition metal ions in lithium-ion cells.

Transition metals are largely believed to dissolve from cathode materials in the +2 oxidation state (e.g., Ni²⁺, Mn²⁺, Co²⁺).^{5,22,28,39–42} These species are paramagnetic, enabling the use of paramagnetic NMR spectroscopy and, in some cases, EPR spectroscopy. In NMR, the nuclei that are located near paramagnetic species undergo rapid relaxation, permitting an indirect understanding of the coordination sphere of the paramagnetic centre; in pulsed EPR, the paramagnetic centre is probed directly and the surrounding magnetic nuclei affect the resonance conditions. In EPR spectroscopy, transition metal ions are often difficult to study if they have integer spins, as is the case for the d⁸S = 1 Ni²⁺ ion, because of the zero field splittings and short relaxation times. In contrast, dilute high-spin d⁵ Mn²⁺ ions are well-suited for EPR studies, because the half-filled outer d shell ensures a small-to-zero ground-state orbital angular momentum, especially for cubic symmetry,⁴³ and small zero field splittings for the central transitions of the S = 5/2 spin system. Consequently, anisotropies are small and electronic relaxation times are comparably long, since modulations in the spin–orbit couplings no longer drive significant relaxation effects.⁴⁴ Provided electronic relaxation times T_1,2e are sufficiently long, pulsed EPR techniques probing small electron–nuclear (hyperfine) interactions are particularly useful for ligand identification. These specialised techniques are necessary since inhomogeneous EPR line broadening typically exceeds the resolution limit necessary to identify hyperfine couplings in transition metal ion complexes apart from those involving the central nucleus. Measurements are performed on frozen solutions at cryogenic temperatures to ensure sufficiently long T_1,2e as well as rigid and potentially well-defined complexes. Although dynamics, as probed by NMR spectroscopy, are inaccessible as a consequence, an ensemble of structural snapshots should be generated during freezing, which should capture the thermodynamic ground state.

We have previously shown that the presence of dissolved paramagnetic ions can result in differential enhancement of the NMR relaxation rates of the signals from a variety of species in both pristine and degraded battery electrolyte solutions.^23,24 We have used these relaxation rates, coupled with the accompanying bulk magnetic susceptibility shifts, to quantify metal concentrations and investigate binding to different electrolyte solvents. We have also used EPR to, for example, establish the solvation shell of dissolved vanadyl ions.^45,46 Here, we combine the direct observation of the paramagnetic ions by EPR with solution NMR measurements to characterise the solvation shells of both Mn²⁺ and Ni²⁺ in pristine battery electrolytes. Model Mn(TFSI)₂ and Ni(TFSI)₂ solutions with varying concentrations of EC, LiPF₆, and LiTFSI are investigated.

Static Mn²⁺ complexes are examined at low temperature in frozen solutions via EPR, field-swept echo and electron nuclear double resonance (ENDOR) experiments, revealing their complex symmetry and the magnetic nuclei in surrounding molecules. In liquids, at ambient temperatures, Mn²⁺ and Ni²⁺ solvation is studied dynamically via¹H and ¹⁹F longitudinal and transverse nuclear relaxation rates, and the favourability of coordination to EC vs. PF₆⁻ in pristine electrolyte solutions is probed. Interactions with PF₆⁻vs. TFSI⁻ are then explored to determine whether M(TFSI)₂ salts, added to mimic the effects of transition metal dissolution, alter the transition metal coordination shells. While transition metals are unlikely to accumulate in the electrolyte solution of a full cell undergoing charge–discharge cycling in the concentrations used in this work (1–8 mM vs., for example, the ∼0.1 mM Ni concentrations found in electrolytes from cycled LiNi_0.8Mn_0.1Co_0.1O₂ cells),^47,48 we consider the concentrations used in this work to be small enough to be representative of the solvation shell of metals dissolved at very low concentrations. That is, the metals are present in low enough concentrations that the pristine electrolyte components are still in significant excess, and no metal clustering effects or other high-concentration effects should occur. We also note that soaking of cathode materials in battery electrolytes, often used to assess dissolution, can cause Mn or Ni accumulation in the electrolyte beyond what is typically observed in cycling cells—particularly in high-temperature soaking experiments, for example, with LiMn₂O₄ or LiNi_0.5Mn_1.5O₄.^42,49–51 While this work centres on coordination in pristine lithium-ion battery electrolytes, the approaches developed and employed herein are suitable to probe Mn²⁺ dissolution in any electrolyte solution or cell chemistry.

Methods

Electrolyte solutions

Transition metal bis(trifluoromethane)sulfonimide (TFSI) salts were added to electrolytes to mimic dissolved transition metals: Mn(TFSI)₂ (Solvionic, 99.5%) and Ni(TFSI)₂ (Alfa Aesar, ≥97%). Ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were used as electrolyte solvents. Solutions used in this work include: 1 M LiPF₆ in 3 : 7 EC : EMC (for EPR and NMR); 1 M LiTFSI in 3 : 7 EC : EMC (for EPR and NMR); 1 M LiPF₆ in EMC (for NMR); and 3 : 7 EC : EMC (for NMR). All solvent ratios are given as volume ratios (v/v). Solutions were prepared in an argon glovebox. For EPR experiments: Solutions were mixed in-house, using LiPF₆ (Sigma Aldrich, ≥99.99% trace metals basis), LiTFSI (Alfa Aesar, 98+%), EC (Sigma Aldrich, 99%, anhydrous), and EMC (Sigma Aldrich, 99%). Mn(TFSI)₂ was dried at 110 °C under dynamic vacuum for three days. For NMR experiments: 1 M LiPF₆ in 3 : 7 EC : EMC was sourced premixed (soulbrain MI PuriEL R&D 280); 3 : 7 EC : EMC was sourced premixed (soulbrain and Solvionic); 1 M LiTFSI in 3 : 7 EC : EMC was mixed in-house (99.95% LiTFSI, Sigma Aldrich, with premixed EC : EMC); and 1 M LiPF₆ in EMC was mixed in-house (99.99% LiPF₆, Solvionic; 99.9% EMC, Solvionic). Salts were dried at 100 °C under vacuum before use.

EPR spectroscopy

Samples for EPR experiments were freshly prepared by dissolving 8 mM of Mn(TFSI)₂ in the electrolyte stock solution. The obtained solution was transferred into a 2 mm outer diameter EPR tube (Wilmad, CFQ) for X-band and into a 0.9 mm outer diameter tube (Wilmad, Suprasil) for Q-band experiments. Tubes were sealed and transferred from the glovebox into the pre-cooled EPR resonator within 15 min after preparation.

EPR experiments were conducted on a Bruker ElexSys E580 spectrometer at a temperature of 20 K, maintained within a helium cryostat (Oxford Instruments, CF935). The EPR resonator was pre-cooled before inserting the sample, to rapidly cool (flash-freeze) the sample. (It should be noted that EPR/ENDOR experiments performed by more slowly cooling the resonator over approximately 15 minutes did not yield observable differences.)

Microwave pulses were amplified using a 1 kW travelling-wave tube amplifier, radiofrequency pulses with a 150 W amplifier. Measurements were performed using an EN4118X-MD4 resonator for X-band and an EN5107-D2 resonator for Q-band microwave frequencies. Field-swept pulsed EPR spectra were obtained by integration in a window of 62 ns centred at the Hahn echo. For X-band, τ = 180 ns and the pulse durations were t_π/2 = 12 ns and t_π = 24 ns. For Q-band, τ = 160 ns and pulse durations were t_π/2 = 20 ns and t_π = 40 ns. A two-step phase cycle was applied. Davies-type electron nuclear double resonance (ENDOR) experiments were conducted at Q-band using the pulse sequence π–D–π/2–τ–π–τ-echo for the microwaves with t_π/2 = 100 ns, t_π = 200 ns, and τ = 450 ns. During the delay D, radiofrequency π-pulses (at close to the ¹H Larmor frequency) lasting 10 μs were applied. ENDOR experiments were acquired at a frequency offset that corresponds to the maximum of the low-field signal, corresponding to the ⁵⁵Mn nuclear spin manifold m_I = −5/2.

EPR spectra simulation

EPR spectra simulations were performed by using EasySpin v6.0.0-dev.49^52,53 running in Matlab v2020b (MathWorks). ¹H powder Davies ENDOR spectra were simulated taking into account all specified nuclei with hyperfine tensor components as obtained from DFT calculations. Easyspin's simulation module salt was used, applying second-order perturbation theory and the product rule. The obtained spectra were multiplied with the Davies ENDOR detection function^54,55with the pulse duration t_π = 0.2 μs of a selective π pulse, the applied RF frequency ν_rf and the ¹H Larmor-frequency ν_1H. Ultimately, a Gaussian convolution (FWHM = 0.1 MHz) was applied.

Field-swept echo-detected spectra were fitted using the EasySpin fitting module pepper. The fit was initialised using an electronic spin 5/2 exhibiting an isotropic g tensor and an anisotropic zero-field splitting, coupled to a ⁵⁵Mn nucleus with isotropic hyperfine coupling. A convolutional Gaussian with a full width at half maximum (FWHM) of 6 MHz for X-band and 13 MHz for Q-band was used along with anisotropic Gaussian zero-field splitting parameter strain.

NMR spectroscopy

For variable temperature NMR, ¹H and ¹⁹F NMR relaxation times were measured on a Bruker Avance III HD 500 MHz spectrometer using a broadband observe (BBO) probe. A sealed capillary of C₆D₆ was used for field locking. Longitudinal, T₁, relaxation times were measured using the inversion recovery pulse sequence and transverse, T₂, relaxation times were measured using the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence^56,57 with 2 ms echo spacings. Experiments were performed at 0, 15, 30, 45, and 60 °C.

Ambient temperature relaxation experiments were performed on a Bruker Avance III HD 300 MHz spectrometer equipped with a Bruker double-channel MicWB40 probe. No deuterated solvents were incorporated into the electrolyte samples. T₁ values were measured using inversion recovery and T₂ values were measured using CPMG, with 2 ms echo spacings (τ) for diamagnetic and Ni²⁺-containing solutions. Due to fast relaxation, for Mn²⁺-containing solutions τ = 0.05–2 ms was used. Use of shorter τ values did not significantly impact the T₂ measurements. For all NMR experiments, J-Young NMR tubes were filled and sealed in an argon glovebox.

Viscosity measurements

Kinematic viscosities of EMC, 3 : 7 EC : EMC (v/v), 1 M LiPF₆ in EMC, 1 M LiPF₆ in 3 : 7 EC : EMC, and 1 M LiTFSI in 3 : 7 EC : EMC were measured in an argon glovebox with a Micro-Ostwald viscometer, type 51610/1 (Xylem Analytics), with an instrument constant K = 0.01063 mm² s⁻². A volume of 2 mL of each solution was used for viscosity measurements. Temperature varied from 25.7–27.2 °C. Kinematic viscosities were converted to dynamic viscosities by multiplying by solution density; solution density was measured by weighing 1 mL of solution in an argon glovebox.

DFT calculations

The density functional theory (DFT) calculations used the software package ORCA, version 5.0.1.⁵⁸ Geometry optimisation was performed on [Mn(EC)₄]²⁺, [Mn(EC)₅PF₆]²⁺, and [Mn(EC)₆]²⁺ (identified as being the likeliest candidate in a set of DFT calculations described in the ESI†), with the TPSSh^59,60 hybrid functional, approximating the relativistic Hamiltonian with the zeroth-order regular approximation (ZORA).⁶¹ Def2-TZVP(-f) basis sets⁶² in their ZORA-recontracted version⁶³ were used. Decontracted def2/J auxiliary basis sets⁶⁴ were employed for the resolution-of-identity and chain-of-spheres approximation.⁶⁵ Convergence was attained by a tight self-consistent-field criterion. Hyperfine coupling tensors were calculated with the hybrid functional TPSSh and the ZORA relativistic approximation, as described recently.^45,46 ZORA-def2-TZVP(-f) basis sets were modified through full decontraction of s shells and adding three more Gaussians, with exponents calculated by multiplying the steepest original primitive with 2.5, 6.25, and 15.625.⁶⁶ Spin–orbit coupling was taken into account with the spin–orbit mean-field approximation (SOMF).⁶⁷ To increase integration accuracy, the defgrid3 setting was chosen with radial accuracy IntAcc further increased to 11 for manganese and 9 for all other atoms.

NMR relaxation theory

In this work, Solomon–Bloembergen–Morgan (SBM) theory is applied to interpret measured NMR relaxation times.^68–72 Within this theory, relaxation is considered to be driven by a dipolar term (the Solomon equations)⁷⁰ and a contact term (the Bloembergen equations).⁷¹ The dipolar term treats the through-space coupling of the nucleus and unpaired electron as an interaction between two point dipoles, i.e., the unpaired electron is localised on the paramagnetic ion, while the isotropic Fermi contact term results from the unpaired electron density present at the nucleus.^68,69 The relaxation of a nucleus bound or close to a paramagnetic metal centre, M, is described bywhere T_1M and T_2M indicate the paramagnetic contributions to the longitudinal and transverse relaxation times and 1/T_1M = R_1M and 1/T_2M = R_2M the relaxation rates. Other terms are the permeability of a vacuum μ₀; nuclear gyromagnetic ratio γ_I; electron spin g-factor g_e; Bohr magneton μ_B; electron spin S; distance between the nucleus and paramagnetic ion r; correlation time associated with the dipolar interactions τ^dip_c; Larmor frequencies for the nuclear spin, ω_I, and the electron spin, ω_S; contact term of the hyperfine interaction constant A and reduced Planck constant ħ (A/ħ in rad s⁻¹, or A/h in Hz); and correlation time for the contact term τ^con_c.

Molecular rotation results in fluctuations arising from the orientation dependence of the anisotropic dipolar coupling tensor, thus the correlation time for the dipolar term

(τ^dip_c)⁻¹ = τ_r⁻¹ + τ_e⁻¹ + τ_M⁻¹ (4)

incorporates the correlation time for molecular rotation τ_r, the electronic relaxation time τ_e, and the lifetime of the inner sphere complex τ_M, which can also be quantified via the chemical exchange time. The rotational correlation time can be approximated from the viscosity η, temperature T, Boltzmann constant k_B, and the molecular volume, assuming rigid spherical particles, using the Stokes–Einstein equationwhere a is the molecular radius, M is the molar mass, d is the density, and N_A is Avogadro's number. Since the contact term is isotropic, its correlation time τ^con_c incorporates τ_e and τ_M only.

(τ^con_c)⁻¹ = τ_e⁻¹ + τ_M⁻¹ (6)

Within SBM theory, the correlation times τ^dip_c or τ^con_c are generally dominated by whichever is shortest of the contributing correlation times in eqn (4) and (6). For small molecules, like those in non-viscous battery electrolyte solutions, τ_r is short (∼10⁻¹⁰ s),⁶⁸ so τ^dip_c cannot be much longer than ∼10⁻¹⁰ s, even if τ_e and τ_M are very long. However, since τ^con_c is not governed by molecular rotation, and in cases where the electronic relaxation and chemical exchange are very slow, τ^con_c can be very long, so that τ^con_c ≫ τ^dip_c. Unlike R_1M, the expression for R_2M contains terms that are linear in τ^con_c and thus R_2M may become significantly larger than R_1M in this regime.^71,73 The contact term also becomes more important when the nucleus being probed is closer to the coordination site in an inner sphere complex, as this increases the hyperfine interaction.⁷⁴

At the magnetic fields used in this work, the Larmor frequency of the electron spin, ω_S, is very large.^71,75,76 Since Mn²⁺ ions undergo relatively slow electronic relaxation, and thus τ_e is long,⁶⁸ all the terms involving 1/ω_S² are expected to be small and can be ignored to a first approximation.^71,75,77 For Mn²⁺, eqn (2) and (3) can therefore be approximated as

Notably, the simplified expression for R_1M is reduced to only a dipolar term, while the expression for R_2M retains dipolar and contact terms.

In most solutions, NMR nuclei are not statically bound to a paramagnetic centre. The measured relaxation rates R₁ and R₂ depend on the timescale and nature of the exchange between bound and unbound species, and whether inner or outer sphere complexes are formed.^68,74 If we consider a system with rapid exchange between bound and unbound ligands, assuming only an inner sphere relaxation mechanism, then the fast-exchange limit of paramagnetic contribution to the relaxation time is given by R_1p = f_MR_1M and R_2p = f_MR_2M, where f_M indicates the molar fraction of the species being probed that is coordinated to the transition metal. For intermediate exchange rates, τ_M⁻¹, more complicated expressions can be derived. For R₁, the terms are relatively simple, where R_1d is the diamagnetic longitudinal relaxation rate,

Expressions for R₂ are more complicated because they also depend on the hyperfine shift, Δω_M, for the bound species. An intermediate regime exists when τ_M⁻¹ is of the same order of magnitude as Δω_M, and additional linebroadening is observed,where R_2d is the diamagnetic transverse relaxation rate. As τ_M⁻¹ approaches zero, R_1p and R_2p both approach zero for an inner sphere complex. The f_M term accounts for the transition metal concentration, the concentration of the solution species being probed, and the solvation number of the solution species in the transition metal coordination sphere. Notably, the paramagnetic relaxation components R_1p = R₁ − R_1d and R_2p = R₂ − R_2d are directly proportional to f_M. The expressions are further complicated in a multicomponent electrolyte solution, where we must also account for the exchange of several different species in and out of the coordination shell. Lastly, we again note that these equations represent paramagnetic relaxation as entirely arising from inner sphere coordination. Relaxation in the second coordination sphere should be dominated by the dipolar term, but it may depend on both a diffusional correlation time, τ_D, and τ_e.⁶⁸ Correlation times may also vary between inner sphere, outer sphere, and bulk species. Additional chemical shift variations may arise for ¹⁹F of PF₆⁻ in the second coordination sphere, which may break the symmetry of the ion and lead to additional R₂ effects. Since the SBM theory was derived for binary mixtures, an analysis of the investigated electrolyte systems is not expected to be quantitative, yet it provides a robust framework for qualitative conclusions.

Results and discussion

EPR spectroscopy, 20 K

Echo-detected EPR spectra. Frozen solutions of 8 mM Mn(TFSI)₂ dissolved in premixed electrolytes containing 1 M LiPF₆ or LiTFSI in a volumetric 3 : 7 ratio of EC and EMC were studied at a temperature of 20 K. Fig. 1 shows field-swept echo-detected pulsed EPR spectra of the two samples at X- and Q-band. The spectra are dominated by the |−1/2〉 ↔ |1/2〉 transitions of the high-spin d⁵ complex. The six-fold line splitting, clearly evident at Q-band, indicates strong hyperfine coupling to the ⁵⁵Mn nucleus of similar magnitude for both samples. The sets of doublets in between the central transitions are formally forbidden transitions originating from zero-field splitting interactions intermixing m_I states,⁷⁸ indicating deviation from perfect cubic symmetry. Their presence is more evident at X-band since the zero-field splitting is less effectively quenched by the electron Zeeman effect at this lower static magnetic field. The observation of well-resolved Mn-hyperfine splittings is clear evidence for the Mn-ions being diluted in the EC/EMC/LiTFSI/LiPF₆ frozen matrix, consistent with the low concentration of this ion.⁷⁹ Close Mn–Mn proximity in, for example, a precipitated Mn salt would result in exchange interactions and more broadening/disappearance of the hyperfine splittings. The broad featureless outer transitions originate from electron-spin transitions other than the |−1/2〉 ↔ |1/2〉 transitions. The two samples appear to possess very similar electronic interaction parameters, but Mn²⁺ in the LiTFSI electrolyte exhibits a larger line broadening, evident from the reduced relative intensity of the sextet (Fig. S1, ESI†).

Fig. 1 (a) X-band and (b) Q-band experimental pulsed EPR spectra recorded using field-swept Hahn-echoes (black) and spin Hamiltonian fits (red). Mn²⁺ is studied in premixed electrolytes with either LiPF₆ (bottom) or LiTFSI (top) salts. All fits used identical interaction parameters but different line broadening (Table 1).

Magnetic interaction parameters. The pulsed EPR spectra were modelled and fitted (Fig. 1, red traces) with a spin Hamiltonian that incorporates electronic and nuclear spin operators and rank 2 interaction tensors. Values for g and A(⁵⁵Mn) could be extracted more accurately via the Q-band rather than the X-band spectra. Identical interaction parameters and symmetries were obtained for both samples (Table 1 and Fig. S1, ESI†), indicating that the electrolyte salt anion, PF₆⁻ or TFSI⁻, has little effect on the magnetic environment of the Mn²⁺ ion. The electronic g tensor is set to be isotropic because no anisotropy was resolved and the observed minor deviation from the free electron value (Δg = 0.0007) is common for high-spin d⁵ complexes. Similarly, the hyperfine coupling to the central atom A(⁵⁵Mn) is determined to be isotropic, indicating that it largely arises from the Fermi-contact interaction caused by spin polarisation of s shells. The sign of A(⁵⁵Mn) cannot be directly inferred from the data but is typically negative for Mn²⁺ complexes.⁸⁰

Table 1 Extracted electronic parameters from EPR relaxation measurements at 20 K and spectral fitting using the spin Hamiltonian formalism. Fit errors are derived from least-squares fits. Systematic errors might exceed the given uncertainties

	Mn^2+ in 1 M LiPF6 in 3 : 7 EC : EMC	Mn^2+ in 1 M LiTFSI in 3 : 7 EC : EMC
a Fitted with a stretched/compressed exponential: S(t) = exp(−(t/Te)^β) + y0.
From spin Hamiltonian fit
Isotropic g-value	2.0016 ± 0.0002
|A|(^55Mn)| (MHz)	273 ± 5
|D| (MHz)	415 ± 60
|E/D| (MHz)
D, E strain (MHz)	230, 60	330, 120
From relaxation measurements^a
T 2e (μs)	0.37	0.49
β T 2e	1.26	1.29
T 1e (μs)	6.16	8.65
β T 1e	0.79	0.77

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Further extractable parameters describe the zero-field splitting arising from the interaction of multiple unpaired electrons within the same (d⁵) ion, which can be described by the electron-spin Hamiltonian term

S⃑DS⃑ = D(Ŝ_z² − Ŝ²/3) + E(Ŝ_x² − Ŝ_y²) (11)

where the tensor D is expressed by the scalar parameters D and E, representing the axial/tetragonal and rhombic distortion, respectively, with 0 < |E/D| < 0.33. These parameters are affected by the symmetry and type of ligands. A least-squares fit of the experimental spectrum was performed with |D| ≈ 415 MHz, where the intensity and position of the forbidden central transitions are dominant in influencing the fit result. Additional simulations with the aim of reproducing the outer transitions reveal that |D| might be larger, and approximately 500–600 MHz (Fig. S2, ESI†). Imperfect fitting can result from a superposition of several similar ligand spheres/conformers of Mn²⁺ complexes or from field-dependent relaxation time dispersion. Conclusively, however, |D| is determined to be on the order of several hundred MHz. Furthermore, |E/D| = 0.32 was extracted from the least-squares fit. Despite considerable uncertainty, large rhombicity is indicated by intensity ratios and positions of forbidden transitions in the regions of low- and high-field central transitions, which are sensitive to |E/D| (Fig. S3, ESI†).

Predominantly, Mn²⁺ complexes are sixfold coordinated in solution, while occasionally five- or sevenfold, and fourfold coordination can occur if halogens and oxygen are directly coordinated.^81,82 For sixfold coordinated Mn²⁺ with close to octahedral symmetry, D is often comparably small, on the order of |D| ≈ 300 MHz or smaller.⁴³ The experimental value extracted here falls in the upper limit of that range, but is at least an order of magnitude smaller than values for known fivefold oxygen coordination or fourfold halogen/oxygen coordination.⁸¹ The exact value of D will depend on the orientation of the ligands,⁸³i.e., degrees of freedom along dihedral angles, the composition and freezing behaviour of the solvent mixture,⁷⁸ and the exact geometry, particularly the bond length between the transition metal and the first ligand atom.⁸⁴ Furthermore, the experimentally determined zero-field splitting tensor exhibits large rhombicity. This rhombicity may also result from different participating ligands in the first coordination sphere. Assuming symmetric sixfold carbonate coordination of Mn²⁺via oxygen following the arguments above, then solvent-separated anions surrounding the complex may also be responsible for the rhombicity. One charge-compensating anion situated around the central atom would favour axial symmetry, as would two with anion–Mn²⁺–anion angles of 180°; two anions with 90° angles would favour rhombic symmetry. The only extractable difference from the pulsed EPR spectrum between the two samples under investigation is the estimated breadth of D and E distribution, given as strain parameters. These are significantly higher for the sample involving TFSI⁻ anions. Again, assuming a solvent-separated ion pair, the O_h symmetry of the PF₆⁻ anion may result in a more ordered inner and outer coordination sphere compared to the TFSI⁻ anion with lower symmetry and a flatter total energy landscape. The coordination of the anions is explored further below via double resonance EPR experiments.

Electronic relaxation. Electronic relaxation times were estimated using the Hahn-echo and inversion recovery pulse sequences for the spin–spin (T_2e) and spin–lattice (T_1e) relaxation times, respectively. Fitting was performed with a stretched/compressed exponential function, the Hahn-echo traces exhibiting a compressed exponential behaviour with a stretching exponent β_T2e > 1, characteristic for Gaussian relaxation often caused by dipole–dipole interactions. However, this is also characteristic for pulsed EPR when a limited excitation bandwidth triggers apparent relaxation effects such as spectral and instantaneous diffusion.⁸⁵ In contrast, the inversion recovery traces exhibit a stretched exponential behaviour with β_T1e < 1, indicating a distribution of relaxation times, which could be caused by a distribution of conformers or varying ligand combinations. For the sample containing TFSI⁻, the T_1e and T_2e values are 40% and 30% longer, respectively, than the values extracted for the sample containing PF₆⁻. This could be due to magnetic nuclei of PF₆⁻ being located closer to Mn²⁺ and/or due to more residual motion of the smaller, more symmetric PF₆⁻ anion as compared to TFSI⁻.

Ligand identification. To study hyperfine interactions with magnetic nuclei in the first- and second-shell solvation spheres, Davies-type electron nuclear double resonance (ENDOR) spectroscopy at Q-band was applied. Hyperfine couplings of ligand nuclei are typically small compared to their respective nuclear Larmor frequency ν_I, and the resonances appear centred around the ν_I specific to each nucleus, exhibiting a powder-like spectral pattern affected by the anisotropy and asymmetry of the hyperfine tensor. The ENDOR experiments reveal couplings to ¹H and ¹⁹F nuclei for both samples (Fig. 2). No couplings to ^6,7Li, ¹⁴N, or ³¹P were detected, neither in ENDOR nor in additional experiments using other hyperfine-targeted techniques (Fig. S4, ESI†). This implies that these nuclei are likely not within roughly 0.5 nm of the central manganese nucleus. The experimental ENDOR spectra reveal that the largest contribution stems from coupled ¹H exhibiting a hyperfine coupling of 0.39 MHz, extracted from the local maxima frequency difference, with major shoulders extending up to around ±1 MHz and minor shoulders up to around ±2 MHz. Resonances centred around ν_19F are weaker suggesting a significantly smaller quantity of surrounding ¹⁹F than ¹H nuclei, even after taking into account signal attenuation of small hyperfine couplings due to a finite microwave pulse length.^54,55

Fig. 2 Experimental (black) and simulated (using values extracted from DFT calculations, blue) Davies ENDOR spectra at Q-band microwave frequencies. Excitation ν_rf was performed at the low-field maximum of the field-swept spectrum shown in Fig. 1b. The radiofrequency (rf) axis is shifted to place the ¹H or ¹⁹F Larmor frequency at 0 MHz. The dashed line indicates the ¹⁹F Larmor frequency at −2.96 MHz (relative to that of ¹H). Lines at −0.17 and 0.22 MHz are a guide to the eye and centred at the experimental ENDOR maxima. On the left, additional ¹⁹F Davies ENDOR measurements are shown where the rf axis is shifted to the ¹⁹F frequency. Interpolation with a cubic spline was used to extract the two local maxima and their frequency difference Δ.

Given that the ENDOR experiments revealed hyperfine coupling to ¹H, DFT calculations of Mn²⁺ coordinated to either EC or EMC in fourfold or sixfold complexes were performed, similar to the recent work of some of the authors.^45,46 Coupling to ¹³C and ¹⁷O were ignored due to their low natural abundance. Spectra were simulated (Fig. 2, blue traces; Table S2, ESI†) using couplings extracted from the calculations, the simulations representing cumulative contributions from all involved ¹H nuclei (4 per EC, 8 per EMC), where some conformational variability is intrinsically incorporated through multiple ligands and nuclei. Overall, all the simulated spectra cover a range that is of the same order of magnitude as the experimental spectra. The experimental maxima are best reproduced by EC ligands, but we cannot distinguish between fourfold or sixfold coordination. The shoulders may originate from coordinating EMC, where the flexible side chains can move closer to the central manganese, increasing the hyperfine coupling. However, the shoulders can also be caused by asymmetric strain which is not included in the simulation, other preferred conformers due to effects from outer solvation spheres, or minor impurities like H₂O ligands which would give rise to intense resonances at around ±1.5 MHz.

Values for the hyperfine couplings of a directly coordinating PF₆⁻ or F⁻ ion of larger than 10 MHz were estimated from additional DFT calculations (simulations in Fig. S5; parameters in Table S3, ESI†). Thus, the experimental spectra are not consistent with a contact ion pair and the ¹⁹F nuclei are rather located in anions in the outer solvation sphere. This means that the Fermi-contact interaction can be neglected for ¹⁹F, leaving a purely dipolar hyperfine tensor. Additional scans in the ν_19F region reveal weak but distinct local maxima (Fig. 2, left panel) from which an approximate ⁵⁵Mn–¹⁹F distance can be estimated using Δ = T, where T describes the diagonal hyperfine tensor A_19F,diag = (−T, −T, 2T). A distance of 6.9 Å for the sample with PF₆⁻ anions and 7.8 Å for the sample with TFSI⁻ anions is extracted from this tensor. By comparison with the DFT geometry optimised carbonate complex, this distance can be assigned to an anion in the second solvation shell – i.e., the cation–anion distance in an outer sphere complex (Fig. S6, ESI†). The presence of two or more anions in the second coordination shell may be responsible for the D/E ratio of >0 estimated above from the simulations of the EPR spectra (in Fig. 1). While there may be further, more distant ¹⁹F ions, resonances with smaller splittings will be even further attenuated, and this estimated distance is likely a good approximation for the closest ¹⁹F nuclei.

NMR spectroscopy, 0–60 °C

The EPR measurements provide compelling evidence for no inner-sphere coordination of anions in frozen Mn²⁺ complexes. We now use variable temperature (VT) NMR spectroscopy at 11.7 T to probe solution dynamics in the liquid state. Fig. 3 shows VT relaxation measurements for electrolyte solutions that are either diamagnetic or that contain 1 mM Mn²⁺. The longitudinal and transverse relaxation rates of EC (¹H) and PF₆⁻ (¹⁹F) are shown at 0, 15, 30, 45, and 60 °C. The R₁ and R₂ values of diamagnetic solutions are labelled R_1d and R_2d, respectively; for paramagnetic solutions, the average R_1d and R_2d values are subtracted from measured R₁ and R₂ values to yield R_1p and R_2p values, i.e., isolating the relaxation enhancement.

Fig. 3 ¹H and ¹⁹F NMR relaxation rates of (a) diamagnetic and (b) paramagnetic solutions of 1 M LiPF₆ in 3 : 7 EC : EMC; measurements were performed at a field strength of 11.7 T. Paramagnetic solutions contained 1 mM Mn(TFSI)₂. Diamagnetic ¹H EC relaxation, shown in panels (a.i) and (a.ii), contains data from one run, while all other panels contain data from two or three runs.

In the diamagnetic solutions, all ¹H EC and ¹⁹F PF₆⁻R_1d and R_2d values decrease (become slower) with increasing temperature (Fig. 3a). This decrease with increasing temperature and thus decreasing viscosity, indicates that both ¹H EC and ¹⁹F PF₆⁻ relaxation occur in the fast motion regime. This is consistent with the relatively low viscosity of the electrolyte solutions that are optimised for Li⁺ mobility. Indeed, variable temperature relaxation measurements of a similar electrolyte solution, LiBF₄ in propylene carbonate, have similarly shown that BF₄⁻ and propylene carbonate are in the fast motion regime.⁸⁶ For the Mn²⁺-containing solutions, the ¹H R_1p and R_2p values similarly decrease with temperature, indicating that the dynamics that drive relaxation are in the fast regime. The ¹⁹F R_1p values initially increase between 0–30 °C then decrease between 30–60 °C. Therefore, at ambient temperature, the relevant mobility process for ¹⁹F relaxation appears to be in an intermediate regime, with τ_c of the same order of magnitude as the inverse of the ¹⁹F Larmor frequency (i.e., 3.4 × 10⁻¹⁰ s)⁸⁷ at 11.7 T, whereas the correlation times driving ¹H relaxation are shorter and in the fast regime. The PF₆⁻τ_c value at the R_1p maximum is consistent with values of τ_r and τ_d of ∼10⁻⁹–10⁻¹⁰ s estimated for low viscosity electrolytes.⁶⁸T_1e values of 6–9 × 10⁻⁶ s were measured for the Mn²⁺ spins at 20 K; while the T_1e values are likely shorter at ambient temperature, τ_e should be the same for EC and PF₆⁻, i.e., it should not account for the differences in the observed correlation times for ¹H and ¹⁹F, suggesting that it is caused by different rotational processes or τ_M values arising from different binding and exchange processes.

The ¹⁹F R_2p values reach a minimum at 45 °C and then increase again. This suggests that the ¹⁹F PF₆⁻ paramagnetic relaxation may be exchange-limited in the probed temperature range. Notably, as the different correlation times typically show a different temperature dependence, the dominant contribution may change as the temperature is altered, with R_1p and R_2p affected differently. Additionally, if exchange is in the intermediate regime, then R_2M can be strongly enhanced. According to the SBM model and as discussed further below, these results imply that the R_2M values likely contain both dipolar and contact terms, while R_1M is dominated by the dipolar term. A more detailed analysis of the variable temperature ¹⁹F data is presented in the ESI,† since it does not unambiguously identify the driving forces for relaxation and rather motivates further relaxation experiments described in the next section.

NMR spectroscopy, ambient temperature

In this section, we separately analyse the trends in the relaxation rate of each environment as the solution composition is changed. We note that ambient temperature measurements were performed at 7.05 T, while VT NMR was performed at 11.7 T. The coordination of both Mn²⁺ and Ni²⁺ is investigated; unlike with pulsed EPR, the rapid Ni²⁺ electronic relaxation and its integer spin does not prevent NMR measurement. Coordination to EMC is not explored directly. We note that metal–solvent coordination is thought to primarily involve EC, because: (i) the solvent properties of EC suggest it is more coordinating than EMC, based on their respective dielectric constants⁸⁸ and solvent polarity parameters (E_T(30) and E^N_T);⁸⁹ and (ii) Li⁺ coordination studies clearly show that EC is preferred over EMC.^7,9,13,15,17 Additionally, a computational study of Mn²⁺ coordination has shown that EC is preferred over EMC.²¹ That is not to say that no coordination to EMC can occur: our previous NMR studies of 1 M LiPF₆ in 3 : 7 EC : EMC containing dissolved Mn(TFSI)₂ showed a larger ¹H hyperfine shift for the EC resonance than for all EMC resonances, suggesting that while coordination to EC is likely preferred, coordination to EMC is also possible.²² Some variation likely exists among the coordination environments of paramagnetic ions, with several possible solvation environments of varying probabilities—but the fraction of Mn²⁺ or Ni²⁺ coordinated to EMC is probably small (or the fraction of time that an EMC molecule spends coordinated to a transition metal is small). Ambient temperature EMC relaxation data are, however, provided in the ESI.†

Viscosity measurements. To explore the coordination between transition metal ions and the different electrolyte components via NMR, solutions were prepared with constant transition metal concentrations but with varying concentrations of LiPF₆, EC, and LiTFSI. Notably, changing the concentration of these components affects the overall solution viscosity. The nuclear relaxation behaviour of both diamagnetic and paramagnetic solutions depends in part upon the rotational and diffusional correlation times, and both the rotation of the different complexes and the diffusion of the various species in solution varies with viscosity (eqn (5)). Kinematic viscosities were therefore measured for diamagnetic electrolyte solutions studied in this work (any viscosity change due to the addition of 1 mM Ni(TFSI)₂ or 1 mM Mn(TFSI)₂ is assumed to be negligible). Densities were also measured or extracted from the literature and used to convert kinematic viscosities to dynamic viscosities. These values are presented in Table 2.

Table 2 Ambient temperature kinematic viscosities, densities, and dynamic viscosities of the electrolyte solutions used in this work. Error in the kinematic viscosities reflects the standard deviation of three measurements. EMC density (marked *) is a reference value;⁹⁰ all other solution densities were measured. Dynamic viscosities were determined by multiplying the kinematic viscosities by the solution densities

	Kinematic viscosity (mm^2 s^−1)	Density (g mL^−1)	Dynamic viscosity (mPa s)
EMC	0.617 ± 0.003	1.012*	0.62
3 : 7 EC : EMC (v/v)	0.940 ± 0.002	1.11	1.05
1 M LiPF6 in EMC	1.440 ± 0.006	1.10	1.58
1 M LiTFSI in 3 : 7 EC : EMC	2.183 ± 0.005	1.23	2.68
1 M LiPF6 in 3 : 7 EC : EMC	2.518 ± 0.007	1.20	3.03

---

Both the kinematic and dynamic viscosities follow the order: 1 M LiPF₆ in 3 : 7 EC : EMC (v/v) > 1 M LiTFSI in 3 : 7 EC : EMC > 1 M LiPF₆ in EMC > 3 : 7 EC : EMC > EMC. The values presented here are consistent with literature values for the dynamic viscosity of EMC (0.65 cP at 25 °C), 3 : 7 EC : EMC w/w (1.11 cP at 20 °C), and 1 molal LiPF₆ in 3 : 7 EC : EMC w/w (3.05 cP at 20 °C).⁹¹

Nuclear relaxation measurements: coordination to EC and PF₆⁻. Fig. 4 shows the longitudinal relaxation rates of electrolyte solutions as the LiPF₆ or the EC concentration is increased from 0 to 1 M (i.e., 3 : 7 EC : EMC + 0–1 M LiPF₆ and 1 M LiPF₆ in EMC + 0–1 M EC). Solutions contain either no transition metals (diamagnetic) or 1 mM Mn²⁺ or Ni²⁺ (paramagnetic).

Fig. 4 The effect of (a) LiPF₆ and (b) EC concentration on longitudinal nuclear relaxation rates. Panels show (i), (iii) and (v) ¹H EC (circles) and (ii), (iv) and (vi) ¹⁹F PF₆⁻ (triangles) relaxation rates of diamagnetic, Mn²⁺-containing, and Ni²⁺-containing solutions of 3 : 7 EC : EMC (v/v) with 0–1 M LiPF₆ or 1 M LiPF₆ in EMC with 0–1 M EC. R_1d (i) and (ii) indicates the R₁ value of diamagnetic solutions, while R_1p (iii)–(vi) indicates the paramagnetic relaxation enhancement (R_1p = R₁ − R_1d). (Note: 3 : 7 EC : EMC (v/v) contains 4.5 M EC.) All measurements were performed at a field strength of 7.05 T.

For solutions with varying LiPF₆, in the diamagnetic solutions, ¹H EC and ¹⁹F PF₆⁻ relaxation rates both increase as the PF₆⁻ concentration increases (Fig. 4a-i and ii). In the paramagnetic solutions, the ¹H EC R_1p values increase as the PF₆⁻ concentration increases, but the ¹⁹F PF₆⁻R_1p values decrease. As LiPF₆ is added to 3 : 7 EC : EMC, the solution becomes significantly more viscous (Table 2), increasing the rotational and diffusional correlation times of the species in solution. In the fast motion regime, longer correlation times increase the relaxation rate, until an R₁ maximum (or T₁ minimum) occurs.^68,92 In the paramagnetic solutions, the ¹H EC relaxation rates increase as the PF₆⁻ concentration increases (Fig. 4a-iii and v); this change is the same as the diamagnetic solution and again is likely a viscosity effect.

The paramagnetic contribution to the longitudinal relaxation rate, R_1p, is proportional to the fraction of nuclei bound to paramagnetic ions, and it is also dependent on the length of time the nuclei are nearby paramagnetic ions (eqn (9)). The decrease in ¹⁹F PF₆⁻ relaxation rates as the PF₆⁻ concentration increases (Fig. 4a-iv and vi) is consistent with the reduction in the ratio of transition metal ions to PF₆⁻. Assuming the total number of paramagnetic-coordinated PF₆⁻ ions remains constant, as the overall molar fraction of PF₆⁻ grows, the coordinated fraction of PF₆⁻ becomes smaller. For instance, if Mn²⁺ is ordinarily nearby two PF₆⁻ ions, then as the PF₆⁻ concentration changes from 0.05 M to 1 M, in a solution containing 1 mM Mn²⁺, the Mn²⁺-coordinated fraction of PF₆⁻ would decrease from 4% to 0.2%. The PF₆⁻ molecule is less likely, on average, to be bound or in close proximity to a paramagnetic ion, e.g., in a second coordination shell; hence, the overall PF₆⁻ relaxation rate is decreased. The observed trend is not linear, which is ascribed, at least in part, to the simultaneous viscosity increase as LiPF₆ is added to solution. EMC relaxation rates are not discussed in detail here; however, the ¹H EMC relaxation rates do increase in diamagnetic and paramagnetic solutions as LiPF₆ is added, presumably due to the viscosity change (Fig. S8, ESI†).

The ¹H EC and ¹⁹F PF₆⁻ relaxation rates of diamagnetic and paramagnetic solutions were also measured as the EC concentration in solution was increased from 0–1 M, with the LiPF₆ concentration constant at 1 M (Fig. 4b). Increasing the EC concentration increases the solution viscosity (Table 2): thus, in the diamagnetic case, the ¹⁹F PF₆⁻ relaxation rates increase (as do the ¹H EMC relaxation rates, shown in Fig. S9, ESI†). However, the diamagnetic ¹H EC relaxation rate decreases as the EC concentration increases. The EC relaxation is therefore controlled by a different (non-viscosity) mechanism, which is likely related to the extent of Li⁺ coordination: while 3 : 7 EC : EMC (v/v) contains 4.5 M EC, the solutions in Fig. 4b contain only 0–1 M EC. Li⁺ is preferentially solvated by EC over EMC,^7,9,13,15,17 but in these solutions, 1 M Li⁺ cannot be fully (tetrahedrally) solvated by EC and there is likely no free EC. Rather, many Li⁺ ions are left to compete for solvation by EC, and with ≤1 EC available per Li⁺, the binding interaction may be stronger than in a solution with a larger EC concentration, as each Li⁺ does not have any other EC molecules to bind to, and the rate of EC exchange in the Li⁺ solvation shell is likely slower. Thus, an EC molecule in a solution of 0–1 M EC + 1 M LiPF₆ on average is more likely to exist as a bound Li⁺–EC complex, relative to an EC molecule in a solution of 4.5 M EC + 1 M LiPF₆ where free EC molecules are also present. Since the Li⁺–EC complex is larger than a free EC molecule, it is associated with longer rotational correlation times than free EC, even though the overall solution is less viscous.

In the paramagnetic solutions, the ¹H EC and ¹⁹F PF₆⁻ relaxation rates both decrease as the EC concentration increases (Fig. 4b-iii–vi), neither being consistent with a (dominant) viscosity-driven inner sphere or an outer sphere mechanism. The decrease in ¹H EC relaxation rate as EC is added to the solution is consistent with both the diamagnetic case, as well as eqn (9), which predicts slower ¹H EC relaxation as the metal : EC ratio changes from 1 : 100 to 1 : 1000 (due to the smaller f_M, the EC spending less time on average bound to a paramagnetic ion). The ¹⁹F PF₆⁻ relaxation time decreases as EC is added, likely because EC is preferentially adopted into the transition metal coordination sphere, and f_M is thereby reduced. When EC is absent, PF₆⁻ spends more time in the transition metal inner coordination shell, resulting in the fastest PF₆⁻ relaxation rates in EC-free solution. By contrast, when the EC concentration is increased further from 1 M to 4.5 M, or 3 : 7 EC : EMC (shown in Fig. 4a, highest concentration point), the ¹⁹F R_1p values drop from 9.7 to 5.6 s⁻¹ for Mn²⁺ and from 0.72 to 0.29 s⁻¹ for Ni²⁺, consistent with the predominance of EC in the Mn²⁺ inner shell seen by EPR.

Taken together, the results in Fig. 4a and b support the EPR results, showing that Mn²⁺ and Ni²⁺ coordinate preferentially to EC over PF₆⁻. Removal of PF₆⁻ from solution affects the EC relaxation rate only little, and it is still dominated by a viscosity effect: if PF₆⁻ coordination were preferred, removing PF₆⁻ would increase the M²⁺–EC fraction, which may produce a faster ¹H EC relaxation rate in LiPF₆-free solution. Instead, the opposite is observed (Fig. 4a-i, iii and v), due to the higher viscosity of LiPF₆-containing solution. By contrast, removing EC from solution causes a dramatic increase in the PF₆⁻ relaxation rate (Fig. 4b-ii, iv and vi), which is notably opposite to what the viscosity change would predict: this indicates that M²⁺–PF₆⁻ coordination is favoured only in the absence of EC, and that the addition of EC reduces the M²⁺–PF₆⁻ fraction.

Fig. 5 shows the R₂/R₁ ratios for all solutions examined in Fig. 4 (an expanded view showing small R₂/R₁ ratios is shown in Fig. S10, ESI†). The R₂/R₁ ratios are small for the ¹H EC resonance for all Mn²⁺-containing samples (1.5–2.1) and for all resonances of the Ni²⁺-containing solutions. In contrast, the R₂/R₁ ratios are consistently large for the ¹⁹F PF₆⁻ resonance in all Mn²⁺-containing samples. When the LiPF₆ concentration is varied (Fig. 5a), the ¹⁹F PF₆⁻R₂/R₁ ratio is ∼8, but when the EC concentration is varied (Fig. 5b), the ¹⁹F PF₆⁻R₂/R₁ ratio decreases linearly from 63.1 to 35.3 as the EC concentration increases to 1 M; it drops further to 9.0 when EC is present at 4.5 M (Fig. 5a, highest concentration point).

Fig. 5 R ₂/R₁ ratios of (a) solutions of 3 : 7 EC : EMC (4.5 M EC) with 0–1 M LiPF₆ or (b) solutions of 1 M LiPF₆ in EMC with 0–1 M EC. Solutions are diamagnetic or contain Ni²⁺ or Mn²⁺; relaxation of ¹H EC (circles) and ¹⁹F PF₆⁻ (triangles) is shown. Expanded view of small R₂/R₁ ratios is shown in ESI,† Fig. S10. Measurements were performed at a field strength of 7.05 T.

The ambient temperature NMR measurements were performed at a lower field strength of 7.05 T, compared to 11.7 T for VT NMR measurements. As a result, ¹H relaxation rates are now more distinctly within the fast regime, while the ¹⁹F R₁ rates are also shifted towards the fast regime, since R₁ maxima are now expected at lower temperatures at the lower field. Assuming that the ¹⁹F relaxation is close to a maximum for 1 M LiPF₆ in 3 : 7 EC : EMC—and in the fast regime—then if the EC concentration is decreased, which should reduce viscosity and τ_r, R_1p is predicted to decrease. Instead R_1p is seen experimentally to increase (Fig. 4), which we ascribed earlier to the decrease of EC bound to the paramagnetic ions in the inner coordination shell. Furthermore, the LiPF₆¹⁹F R₂/R₁ ratio increases noticeably from 9 in 3 : 7 EC : EMC electrolytes to 63 in EMC-only electrolytes (Fig. 5). In an outer sphere mechanism, R₁ and R₂ should approach each other in the fast regime. This suggests that a driving force (fluctuating field) beyond simply an outer-shell dipolar mechanism plays an increasingly important role, at least at low EC concentrations. We suggest that inner sphere mechanisms start to play a larger role; this is unsurprising because less EC is present in the Mn²⁺ inner shell.

In an inner sphere complex, Mn²⁺ will coordinate directly to ¹⁹F, resulting in more electron density at the nucleus being studied, and a large hyperfine interaction (static average DFT value of A_iso(¹⁹F) = 2.90 MHz including Mn–F–P, Table S3, ESI†). Thus, at lower EC concentrations, at least, we tentatively suggest the large R₂/R₁ ratios may arise from a contact contribution to relaxation for ¹⁹F. Rotations of the coordinating PF₆⁻ anion, leading to changes in the F atoms that are coordinated to Mn²⁺, will also result in large fluctuations of the ¹⁹F hyperfine interactions, providing another (contact) relaxation mechanism. By contrast, with EC, Mn²⁺ coordinates at the carbonyl oxygen,^20,21 which is located much farther away from the ¹H nuclei that are being studied, resulting in a small hyperfine interaction as is seen in the EPR results (static average DFT value of A_iso(¹H) = 0.03 MHz, Table S2, ESI†) and thus a small contact term for ¹H. The observed small ¹H R₂/R₁ ratios are consistent with this.

Nuclear relaxation measurements: coordination to TFSI⁻. Measurements to assess the degree of coordination between the transition metal cations and the TFSI⁻ counterion were then performed. Fig. 6 shows the effect of incrementally replacing LiPF₆ with LiTFSI; the salt concentration in solution is 1 M in total, comprising either 0%, 25%, 50%, 75%, or 100% LiTFSI, with the remainder of the salt content comprising LiPF₆.

Fig. 6 (a), (d) and (g) ¹⁹F TFSI⁻ (squares), (b), (e) and (h) ¹⁹F PF₆⁻ (triangles), and (c), (f) and (i) ¹H EC (circles) longitudinal relaxation rates of solutions of 3 : 7 EC : EMC (v/v) with 0–1 M LiPF₆ and 1–0 M LiTFSI, where the total Li⁺ concentration remained constant at 1 M. Solutions were diamagnetic or contained 1 mM Mn(TFSI)₂ or Ni(TFSI)₂.

In Fig. 6a–c, the diamagnetic relaxation rates all decrease as the LiPF₆ salt is replaced by LiTFSI; this is consistent with the viscosity measurements of these solutions in Table 2 showing that the LiPF₆ solution is more viscous. When the solution contains 1 mM Mn²⁺, the ¹H EC relaxation rates decrease (Fig. 6f), again matching the diamagnetic, viscosity-driven behaviour. However, the ¹⁹F PF₆⁻ and ¹⁹F TFSI⁻ relaxation rates both increase as LiPF₆ is replaced by LiTFSI (Fig. 6d and e). If Mn²⁺ coordination to PF₆⁻ and TFSI⁻ are equally preferable, there should be no effect, to a first approximation, with changing the LiPF₆/LiTFSI concentration, as the ratio of Mn²⁺-coordinated anions vs. total anions would stay the same. However, if PF₆⁻ coordination is preferred over TFSI⁻, then the ¹⁹F PF₆⁻R_1p and ¹⁹F TFSI⁻R_1p values should both increase as LiTFSI replaces LiPF₆, as is observed. (If PF₆⁻ coordination is preferred, further increasing the PF₆⁻ concentration only reduces the total coordinated fraction, lowering PF₆⁻R_1p values.) Notably, this result is inconsistent with previous computational work suggesting TFSI⁻ coordination should be preferred.²¹ However, variable temperature NMR measurements were not performed for TFSI⁻ solutions, and it is possible that the paramagnetic ¹⁹F TFSI⁻R₁ is exchange-limited. If so, then increasing the TFSI⁻ concentration may result in more rapid TFSI⁻ chemical exchange, which could ultimately cause the relaxation rate to increase (viaeqn (9)). In outer sphere mechanisms, since the concentration of metals remains constant, the viscosity of the solutions is expected to dominate this mechanism, the results lending further support to the role that there is an inner sphere contribution to relaxation.

In the Ni²⁺-containing solutions (Fig. 6g–i), the ¹⁹F TFSI⁻, ¹⁹F PF₆⁻, and ¹H EC relaxation rates all decrease as LiPF₆ is replaced by LiTFSI. While these are the same results as observed in diamagnetic solution, this is not explained entirely by viscosity effects: Fig. 4a-vi shows that in a Ni²⁺-containing solution, removing PF₆⁻ (without replacing it with TFSI⁻) does cause the ¹⁹F PF₆⁻R_1p values to increase as f_M increases. The largest ¹⁹F PF₆⁻R_1p value coinciding with the smallest Ni²⁺ : PF₆⁻ ratio (1 : 1000) therefore suggests that Ni²⁺ is only nearby PF₆⁻ when PF₆⁻ is present in substantial concentrations. Additionally, the decrease in ¹⁹F TFSI⁻R_1p values as the TFSI⁻ concentration increases is consistent with a decrease in f_M. Fig. 6 therefore indicates that Ni²⁺ prefers TFSI⁻ coordination over PF₆⁻ coordination, while Mn²⁺ prefers PF₆⁻ coordination over TFSI⁻ coordination. However, in a 1 M LiPF₆ electrolyte solution where 1 mM Mn(TFSI)₂ or Ni(TFSI)₂ is added as a model compound, the PF₆⁻ concentration is 500× larger than the TFSI⁻ concentration and interactions with the PF₆⁻ anion may dominate for both Mn²⁺ and Ni²⁺.

An analysis of the R₂/R₁ ratios for the same solutions as studied in Fig. 6 is provided in the ESI† (Fig. S11); these data are consistent with conclusions drawn from the R_1p data. The idea of preferential coordination to TFSI⁻ by Ni²⁺ but not Mn²⁺ may be rationalised on the basis of crystal field arguments. Ni²⁺ (d⁸) complexes, with larger crystal field stabilisation energies and smaller radii, are more long lived once they form, whereas Mn²⁺ (d⁵) complexes have zero crystal field stabilisation energies, larger radii, and more rapid equilibria (following the Irving–Williams order of stability), i.e., more fluctional, short-lived complexes are formed.^93–95 The more rapid equilibria found for Mn²⁺ ions means that EC molecules will also move in and out of the Mn²⁺ coordination shell, allowing Mn²⁺–PF₆⁻ inner sphere interactions to occur, albeit short-lived, contributing to the more rapid transverse nuclear relaxation. These interactions are weak, however, so that no inner sphere binding is seen in the EPR experiments. Thermodynamically, the M²⁺–TFSI⁻ interaction is likely stronger than the M²⁺–PF₆⁻ interaction, in accordance with computational work for Mn²⁺,²¹ but transition metals may coordinate at any F of the small, symmetric PF₆⁻ molecule, whereas TFSI⁻ is bulkier and may be more difficult to accommodate around a metal ion. In actual electrolyte solutions containing dissolved transition metals and multiple solvent molecules, it may be that the smaller PF₆⁻ is easier to incorporate in the Mn²⁺ solvation shell and thus outcompete TFSI⁻ in this respect. By contrast, the greater crystal field stabilisation of Ni²⁺ should result in stronger binding of TFSI⁻ to Ni²⁺vs. binding to Mn²⁺, combined with longer lived complexes. These results indicate the importance of experimental studies to complement theoretical work. Finally, we note that the timescales in which the anions and solvation molecules move in and out of the solvation shells, and the role that metal binding has on the rotational modes of the anions themselves, will clearly have implications for the different relaxation processes, motivating further molecular dynamics simulations of these systems, coupled with more variable temperature NMR studies.

Conclusions

Electron–nuclear spin interactions have been exploited in this study to identify the solvation sphere(s) of paramagnetic transition metal ions in battery electrolytes, combining synergetic insights from EPR and NMR into ligand identity and dynamics. Pulsed EPR spectroscopy was beneficial for the direct observation of ligands using ENDOR. However, EPR is limited to systems with slow electronic relaxation rates, typically requiring cryogenic measurement temperatures, and is best suited to metals with non-integer electron spins, i.e., Mn²⁺. In contrast, simple T₁ and T₂ NMR measurements at ambient conditions can provide indirect insights into the nature of solvation shells, for a wide range of paramagnetic ions and including dynamic effects such as ligand exchange. This is due to nuclear relaxation rates of electrolyte components being highly sensitive to the presence of paramagnetic transition metals.

In frozen pristine electrolyte solutions, EPR experiments identified that dissolved Mn²⁺ is primarily coordinated to EC in solutions of 1 M LiPF₆ or LiTFSI in 3 : 7 EC/EMC (v/v), with distorted O_h symmetry (sixfold coordination). In addition to EC coordination, rhombic zero-field splitting suggests an asymmetric coordination environment, influenced by EMC and/or salt anions. This is consistent with NMR experiments of Mn²⁺- and Ni²⁺-containing solutions with a variety of salt and solvent concentrations. NMR results showed that EC outcompetes PF₆⁻ in the solvation shell. At the same time, the extremely fast ¹⁹F PF₆⁻ transverse relaxation in Mn²⁺-containing solutions, particularly at low EC concentrations, likely arises from a contact term and indicates that some PF₆⁻ is indeed present in the first solvation shell (i.e., as contact ion pairs). In contrast, ¹⁹F ENDOR experiments do not indicate the presence of a contact ion pair, but instead a solvent-separated ion pair is found, presumably due to the sole presence of the thermodynamically more stable EC-coordinated complex at cryogenic temperature. Taken together, the results suggest that the Mn²⁺ and Ni²⁺ solvation shell in pristine electrolyte solutions comprises primarily EC, with some exchange of PF₆⁻ between the inner and outer spheres, particularly in Mn²⁺ solutions at low EC concentrations. NMR experiments probing coordination to TFSI⁻ showed that TFSI⁻ can displace PF₆⁻ in the Ni²⁺ solvation shell, but there is no clear evidence from either NMR or EPR that this occurs in the Mn²⁺ solvation shell.

These new insights on transition metal coordination add experimental evidence to previous work, which is dominated by computational studies. A clear understanding of metal solvation may contribute to approaches adopted to prevent transition metal dissolution, deposition, and overall battery capacity fade. Finally, the presented combined EPR–NMR approach is well-suited to assess the solvation of paramagnetic transition metals and may be readily applied to any other electrolyte system or cell chemistry, including studying transition metal coordination to novel electrolyte components or electrolyte degradation species.

Data availability

Additional data supporting this article, including DFT xyz files, have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

JPA thanks the Natural Sciences and Engineering Research Council of Canada for funding; JPA and CPG acknowledge support from the Faraday Institution under grant number FIRG001 and from the Royal Society via a professorship to CPG; CS acknowledges computing resources granted by RWTH Aachen University under project rwth0701; HES thanks Prof Stuart Clarke for helpful discussions and the EPSRC for funding, award number EP/R513180/1; EJ and CPG acknowledge support from the ERC grant BATNMR no. 835073.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp01663g

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