Realizing fast Li-ion conduction of Li₃PO₄ solid electrolyte at low temperature by mechanochemical formation of lithium-containing dual-shells

Abstract

Dual lithium-containing hydride/oxide shells are constructed by in situ mechano-induced assembly of Li₃PO₄ reacting with LiBH₄. The ionic conductivity of the Li₃PO₄-based composite is promoted nearly 4 orders of magnitude, up to 0.04 mS cm⁻² at 75 °C as compared to the pure one, possessing an electrochemical window of −0.2–5 V (vs. Li/Li⁺).

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As an efficient energy storage device, rechargeable lithium-ion batteries (LIBs) play an important role in replacing fossil fuels that cause environmental pollution.¹ Commercial liquid lithium-ion batteries are widely used in micro-electronic products, hybrid electric vehicles, grid energy conversion storage systems and other energy storage fields.^2–4 However, they can lead to safety issues as they use volatile liquid electrolytes, which are flammable and explosive.^5,6 At present, it is urgent to develop safe and reliable batteries with high storage energy density, long cycle life and low price. To solve the above-mentioned problem, all-solid-state batteries have been designed where high-capacity lithium metal acts as the anode and the organic liquid electrolytes are replaced by safe and stable solid electrolytes (SSEs).^7,8 The key issues of the electrolyte of all-solid-state batteries depend on the employment of new electrolyte materials, possessing high ion conductivity, and good electrochemical and mechanical stability at room temperature.^9,10

So far, extensive research has been conducted on SSEs covering polymer materials and inorganic materials.^11,12 Among them, the typical conductive polymers are polyoxyethylene (PEO),¹³ polyacrylonitrile (PAN),¹⁴ polymethyl methacrylate (PMMA)¹⁵ and polyvinylidene difluoroethylene (PVDF),¹⁶ and the typical inorganic conductive materials include glass/ceramic,¹⁷ garnets,¹⁸ sulfides,¹⁹ perovskites²⁰ and hydrides.²¹ Despite extensive research and significant progress in solid electrolytes over the past few decades, there are currently no commercially available battery solutions to meet the requirements of production and daily life.^22,23 The main challenge at present is to develop new solid electrolytes with facile preparation processes and excellent electrochemical performance, possessing high lithium ion conductivity.²⁴

As a typical glassy material, Li₃PO₄ takes advantages of low cost, easy preparation, environmental inertness, and negligible electron conductivity.^25–27 Therefore, it has been widely applied in improving the electro-chemical properties of lithium-rich electrode materials²⁸ and inhibiting the growth of lithium dendrites in lithium metal batteries.²⁹ However, the application of Li₃PO₄ in solid electrolytes is barely reported because of its much lower conductivity of 10⁻¹⁰ S cm⁻¹ at room temperature (RT).²⁵

Herein, we propose the technique of surficial modification of Li₃PO₄ particles by introducing lithium borohydride (LiBH₄) to achieve high ionic conductivity and good electro-chemical properties. The LiBH₄ as a fast-ion conductive solid electrolyte was first reported by Matsuo et al., which shows good electro-chemical stability and good interface compatibility with electrode materials.^30,31 It was found that the prepared LiBH₄/Li₃PO₄ composite had a double-coated structure, namely, LiBO₂ as the intermediate layer to prevent further reaction between LiBH₄ and Li₃PO₄, and the amorphous LiBH₄ as an outer layer to provide a continuous ionic conductivity network among Li₃PO₄ particles to synergistically improve their ionic conductivity. The composites presented an ionic conductivity up to 0.04 mS cm⁻² at 75 °C, which is nearly 4 orders of magnitude higher than that of pure Li₃PO₄. Moreover, the solid electrolyte of LiBH₄/Li₃PO₄ composites exhibited excellent electro-chemical properties, such as a wide electro-chemical window (5 V vs. Li/Li⁺) and negligible number of electron migrations. These results provide new possibilities for developing suitable SSEs with excellent ionic conductivity.

The lithium-contained dual shells of Li₃PO₄ electrolyte were assembled by one-step ball milling (Fig. 1a). According to the X-ray diffraction (XRD) patterns in Fig. 1b, the diffraction peaks of Li₃PO₄ keep stable while the peak strength of LiBH₄ gradually weakens with the increase of the milling time from 2 h to 50 h, indicating that the grain size becomes refined. The thermal stability of the 50 wt% LiBH₄/Li₃PO₄ composite as a model was further studied by in situ differential calorimetry. From (Fig. 1c and Fig. S1a, ESI†), it can be seen that endothermic and exothermic peaks are observed on each differential scanning calorimeter (DSC). The endothermic peak at about 110 °C and the exothermic peak at ∼76 °C correspond to the phase transition temperature of LiBH₄ from Pnma to P6₃mc,³⁰ while the exothermic peak observed at ∼280 °C and the endothermic peak observed at ∼270 °C represent its melting and re-crystallization process. The second cycle of differential scanning calorimeter exhibits a similar tendency though a slight shift in temperature curve. These results confirm that the LiBH₄/Li₃PO₄ composites are thermally stable at a temperature range of 308–573 K, which would help the all-solid-state battery maintain stability in a high-temperature environment. Fig. 1(d–i) shows the scanning electron microscope (SEM) images and Energy Dispersive Spectrometer (EDS) mapping of the 50 wt% LiBH₄/Li₃PO₄ composites with the milling time of 2 h and 50 h, respectively. It can be seen that the introduction of LiBH₄ caused obvious agglomeration and adhesion of the particles. On magnified observation, we found that some of the sample particles exhibited an encapsulated morphology, which may be due to the formation of a low Young's modulus LiBH₄ covering layer around high Young's modulus Li₃PO₄ particles.^32,33 This is further confirmed by the EDS mappings of highly dispersed B and P elements.

Fig. 1 (a) The preparation schematic diagram of LiBH₄/Li₃PO₄ composites with a dual-layer structure; (b) X-ray diffraction patterns of the 50 wt% LiBH₄/Li₃PO₄ composite with different milling times; (c) in situ DSC curves of the 50 wt% LiBH₄/Li₃PO₄ composite by heating/cooling; SEM image and EDS mapping of 50 wt% LiBH₄/Li₃PO₄ milled for 2 h (d)–(f) and 50 h (g)–(i), respectively.

To gain a deeper understanding of the sample micro-structure, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) surface sputtering techniques were conducted. From the TEM image and the corresponding selected electron diffraction pattern (Fig. 2a–d), it can be seen that the sample has a dual-layer structure, where the amorphous coating corresponds to LiBH₄, the lattice fringes correspond to the (010) intermediate layer LiBO₂ and the inter-planar spacing of 0.3078 nm belongs to the (111) crystal plane of the Li₃PO₄ core, respectively (Fig. 2d and e). Fig. 2(g and h) show the XPS depth profiling plots of elements B and Li in the samples before and after sputtering, respectively. The peaks located at 187.8 eV and 191.7 eV in the high-resolution B 1s XPS spectrum before sputtering correspond to the B–H bonds³⁴ in LiBH₄ and the B–O bonds in LiBO₂,³⁵ respectively, and the relative intensity of the characteristic peaks of LiBO₂ in the B^1s spectrum is further enhanced after 15 s sputtering. The high-resolution Li 1s characteristic peaks corresponding to LiBH₄ (∼56.2 eV) and LiBO₂ (∼55.2 eV) also appear in the XPS spectrum,³³ and the relative content of LiBO₂ and the relative content of LiBH₄ gradually decrease after sputtering for 15 s. Combining the TEM and XPS results, we found that the cladding layer formed around the Li₃PO₄ particles is a bilayer structure composed of LiBH₄ as the outer layer and LiBO₂ as the intermediate layer. LiBH₄ as an amorphous provides more lithium-ion transmission channels and continuous ion-guided circuits for the Li₃PO₄, while LiBO₂ as the intermediate layer plays a role in preventing LiBH₄ and Li₃PO₄ from continuous reaction in the ball-milling process. Therefore, the formation of the cladding layer not only improves the ionic conductivity of Li₃PO₄ but also enhance their cold-working formability.

Fig. 2 (a) and (b) TEM images and (c) its SAED pattern b, as well as (d)–(f) HRTEM images of 50 wt% LiBH₄/Li₃PO₄ composites; and high resolution XPS spectra of (g) B^1s and (h) Li^1s before and after etching for different times.

In order to determine the Li⁺ ion conductivity, the as-milled LiBH₄/Li₃PO₄ composite was cold pressed into pellets at room temperature and then held for 8 h subjected to electrochemical impedance spectra (EIS) examination. It could be found that the longer the ball grinding time, the smaller the impedance of the sample at room temperature (Fig. 3a). The ion conductivity curves of the sample with different milling times are shown in Fig. 3b. Compared with the 50 wt% LiBH₄/Li₃PO₄ composite material ground for 2 h at room temperature, the ion conductivity of the 50 wt% LiBH₄/Li₃PO₄ composite material ground for 50 h increased by nearly an order of magnitude. In addition, the activation energy (E_a) of lithium-ion mobility can be quantitatively analyzed by linear fitting of the data in Fig. 3b by using Arrhenius eqn (1):³¹

where K_B stands for Boltzmann's constant and C stands for the constant. The Arrhenius calculation results are shown in Fig. 3c. As the milling time prolongs, the activation energy of the sample gradually decreases, which is due to the finer grain size leading to a lower activation energy interface that can provide for lithium-ion conduction. This is also confirmed by the Brunauer–Emmett–Teller (BET) method testing of three sets of samples with different lengths of ball-milling time (Fig. S4, ESI†). As shown in Fig. 3d, the activation energy gradually decreases with extending the milling time, which is an effective way to improve the ion conductivity.

Fig. 3 (a)–(c) Ion transport characteristics of the 50 wt% LiBH₄/Li₃PO₄ composite under different ball milling times; and (d) the relationship between ionic conductivity and milling time.

The direct current polarization curve of SUS|50 wt% LiBH₄/Li₃PO₄|SUS symmetrical batteries was measured at 105 °C. As shown in Fig. 4a, the discharge current drops sharply after applying a 10 mV step voltage and then tends to stabilize. The electron conductivity (σ_e) of the sample is calculated using eqn (2):³¹

where I₀ is the steady-state current, d is the electrolyte sheet thickness, U₀ is the applied voltage, and S is the surface area of the electrolyte sheet. The electron conductivity of the sample was calculated to be about 1.176 × 10⁻⁷ S cm⁻¹, which was about 2 orders of magnitude lower than the conductivity of lithium-ions at the same temperature, and the lithium-ion mobility was determined to be greater than 0.99. This strongly indicates that the 50 wt% LiBH₄/Li₃PO₄ composite solid electrolyte is a typical single-ion conductor and the electron conduction is negligible, which would help to improve the cycling performance of the solid electrolyte and the inhibition of lithium dendrites. Fig. 4b shows that the Li|50 wt% LiBH₄/Li₃PO₄ |SUS battery is swept from the starting voltage −0.2 V to the termination voltage 5 V, with corresponding cyclic voltammetry (CV) curves at 105 °C at a sweep rate of 5 mV s⁻¹ (a total of 5 turns). From the CV curve it can be seen that no other significant red-dotted peaks were observed in the de-embedding lithium current peaks (observed near 0 V in the wide voltage window of −0.5–5 V), which indicates that the 50 wt% LiBH₄/Li₃PO₄ composite solid electrolyte has a wide electro-chemical window.

Fig. 4 (a) DC polarization curve, (b) cyclic voltammetry curve, (c) constant current cycle curve and (d) stepped current density galvanostatic cycle of the 50 wt% LiBH₄/Li₃PO₄ composite.

The cycling performance of the 50 wt% LiBH₄/Li₃PO₄ composite solid electrolytes and the inhibition ability of lithium dendrites were further evaluated. At 105 °C, we applied to the Li|50wt% LiBH₄/Li₃PO₄|Li symmetrical battery a constant current of 0.5 mA/cm⁻² and a step current density of 0.1 mA cm⁻² as the starting current and 0.1 mA cm⁻² as the step length. As is shown in Fig. 4c, the battery has no significant voltage fluctuations within 100 cycles, which proves that the 50 wt% LiBH₄/Li₃PO₄ composite solid electrolyte has good electro-chemical cycling stability after ball-milling for 50 h. In addition, the limited current density of the Li|50wt% LiBH₄/Li₃PO₄|Li battery is determined to be 1.8 mA cm⁻² (Fig. 4d), which corresponds to the critical current density for lithium dendrite formation and the above results indicate that the stable interphase between Li and LiBH₄/Li₃PO₄ can effectively suppress the growth of Li dendrites. This remarkable cycling performance can be attributed to two factors including the stable surface contact among particles due to the filling of the amorphous outer layer of LiBH₄ providing a continuous ionic conductive network between Li₃PO₄ particles and the extremely low electronic conductivity that effectively suppresses the growth of Li dendrites.³⁶

In summary, we developed an effective strategy to improve the lithium-ion conductivity of the Li₃PO₄-based solid-state electrolyte, namely, the LiBH₄/Li₃PO₄ solid-state electrolyte constructed by in-situ mechano-chemical reaction, which exhibits a bilayer structure: (i) the formed LiBO₂ as an intermediate layer will prevent further reactions between LiBH₄ and Li₃PO₄, (ii) the amorphous outer layer of LiBH₄ can provide a continuous ionic conductive network between Li₃PO₄ particles. The LiBH₄/Li₃PO₄ (50 wt%) composite exhibits an excellent ion conductivity of 0.04 m S cm⁻² at 75 °C. This is mainly due to the amorphous LiBH₄ outer layer providing more lithium ion transfer channels and fine particles resulting in lower energy interfaces for lithium ion conduction. In addition, the LiBH₄/Li₃PO₄ electrolyte also exhibits a wide electro-chemical stability window, excellent cycling stability and strong lithium dendrite inhibition. These findings can provide an alternative route to improve the ion conductivity of solid electrolytes in all-solid-state batteries.

This work was financially supported by the Key Program for International S&T Cooperation Projects of China (No. 2017YFE0124300), National Natural Science Foundation of China (No. 51971002, 52171205, 52101249 and 52171197), Scientific Research Foundation of Anhui Provincial Education Department (No. KJ2021A0360, 2108085QE191), Anhui Provincial Natural Science Foundation for Excellent Youth Scholars (No. 2108085Y16) and Scientific Research Foundation of Hunan Provincial Education Department (No. 22B0769).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, DSC curves, EIS, Arrhenius diagram and specific surface area for the LiBH₄/Li₃PO₄ composite. See DOI: https://doi.org/10.1039/d3ma00125c

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