Dominic
Bresser
a,
Stefano
Passerini
*bc and
Bruno
Scrosati
*bde
aCEA-Grenoble, DRF/INAC/SPrAM/PCI, 17 Rue des Martyrs, 38054 Grenoble, France. E-mail: dominic.bresser@kit.edu
bHelmholtz-Institute Ulm (HIU), Electrochemistry of the Battery, Helmholtzstrasse 11, 89081 Ulm, Germany. E-mail: bruno.scrosati@gmail.com
cKarlsruher Institute of Technology (KIT), PO Box 3640, 76021 Eggenstein-Leopoldshafen, Germany. E-mail: stefano.passerini@kit.edu
dElettrochimica ed Energia (EeE), 00199 Rome, Italy
eItalian Institute of Technology (IIT), Via Morego 30, 16163 Genova, Italy
First published on 21st September 2016
The essential need for new lithium-ion battery materials providing higher energy and power densities has triggered an exceptional increase in scientific and industrial research efforts in recent years. Regarding the anode side, the two major research directions to achieve improved energy densities have, so far, focused on materials which can host lithium either by alloying or by a conversion mechanism. Very recently, however, a new class of potential next generation anodes is gaining continuously increasing attention: conversion/alloying materials. Herein, we provide for the first time a comprehensive review on this new materials' class. Initially, we discuss the two possible approaches to realize a combined conversion and alloying mechanism in a single compound, starting either from pure conversion or pure alloying materials. Based on this overview we subsequently highlight the fundamental insights and their potential advantages, which shall provide scientists with some general considerations and principles for the development of new, further enhanced conversion/alloying materials.
Broader contextLithium-ion batteries, commercialized for the first time by SONY Corp. in 1991, have marked a breakthrough in the field of electrochemical energy storage, providing energy and power densities 2 to 10 times greater than any other battery chemistry. For this reason, they are already the technology of choice for portable electronics and also for large-scale applications like electric vehicles and stationary energy storage. Particularly for the latter ones, however, further improvement is needed and after substantial enhancement by engineering-driven advances, including, for instance, the electrode and cell design, the next great leap forward will require the implementation of new chemistries, replacing the state-of-the-art active materials, i.e., lithium transition metal oxides or lithium iron phosphate and graphite on the cathode and anode side, respectively. Alternative cathode materials with great potential are certainly sulfur or oxygen, while research activities targeting the development of alternative anode materials focused initially on metallic lithium, alloying, and conversion compounds. Recently, however, a new class of alternative anodes is attracting continuously increasing attention: conversion/alloying materials, which combine these two reaction mechanisms in one single compound. |
While on the cathode side environmentally benign and abundant materials like sulphur and oxygen hold great promise for realizing next generation batteries with energy densities approaching those of gasoline-powered vehicles,10–15 the utilization of such high energy cathodes consequently calls also for high energy anode materials, as schematically illustrated in Fig. 2. The ideal anode candidate would be, without a doubt, metallic lithium (Fig. 2a). Nonetheless, despite substantial improvements in overcoming interfacial and cycling efficiency issues by using, for instance, ionic liquid-,16,17 polymer-based,18,19 or solid20,21 electrolytes or protecting the lithium metal surface with carbon,22 the severe safety issues related to dendrite growth and its high reactivity still remain to be completely solved.23 For this reason, several groups started to investigate the combination of sulphur- or oxygen-based cathodes with alternative anode materials like silicon24–27 or tin,28,29 which can reversibly alloy with lithium upon discharge (see Fig. 2b for the case of tin). Such alloying materials, comprehensively reviewed very recently by Obrovac and Chevrier,30 generally offer high specific capacities (e.g., 993 mA h g−1 in case of tin and up to 3578 mA h g−1 in case of silicon) and commonly operate at reasonable low potentials, rendering them generally as very attractive alternatives for the state-of-the-art lithium-ion anode graphite (372 mA h g−1).30–34 But their widespread implementation in practical lithium-ion cells is hampered by the extensive volume changes upon (de-)lithiation of up to about 300%.30–34 In an attempt to circumvent this issue, it was proposed to utilize the corresponding oxides (e.g., SnO2), thus benefitting from the initial irreversible formation of a lithium oxide (Li2O) matrix, which buffers the occurring volume variations upon subsequent (de-)lithiation of the simultaneously formed metallic nanograins.35–38
The reversible lithium storage mechanism occurring for the second major class of alternative high capacity anodes, the so-called conversion materials,34,39–44 can, in fact, be considered, as exactly the opposite. In case of the most investigated group of such compounds – transition metal oxides (TMOs; with TM = e.g., Co, Fe, Ni, or Cu) – initially the same reaction occurs, i.e., the reduction of the TMO, forming metallic TM nanograins embedded in an amorphous matrix of lithium oxide.39 Nevertheless, in this case the metallic nanoparticles remain electrochemically inactive and lithium oxide is formed reversibly. Such extensive bond cleavage and re-forming, however, results in a rather wide operational potential range and a large voltage hysteresis between charge and discharge, i.e., a relatively low energy storage efficiency.34,40,41,45 The reaction mechanisms for these classic types of lithium ion storage (including intercalation for graphite) are schematically illustrated in Fig. S1 (ESI†).
In this article, we review for the first time the latest approach for developing high energy lithium-ion anode materials, targeting the beneficial combination of these two lithium storage mechanisms in a single compound, which we will hereinafter refer to as conversion/alloying materials, CAMs. Initially, we briefly survey pure conversion-type compounds with a particular focus on recently reported fundamental insights into the reaction mechanism. Based on this survey, we then present an overview on the first alternative to realize CAMs, i.e., the partial substitution of the electrochemically inactive TM in conversion-type compounds by elements which can further alloy with lithium as, for example, the case for ZnTM2O4 (with TM = Fe, Co, Mn). As a result, these materials provide significantly increased capacities compared to the pure conversion-type ones. In a next step, we introduce the second alternative to further increase the energy density of conversion/alloying materials by reversing this concept, i.e., starting from an alloying compound and enabling the reversible conversion, as realized, for instance, in TM-doped zinc or tin oxide.46–50 This approach, indeed, follows the fundamental insight that the key to enable the reversibility of conversion reactions is the formation of an electronically conductive network of TM nanograins.51–53 In combination with recent findings that some elements like Ge,54 Sn,55 or Zn56 do not only form alloys but moreover potentially enable the reversible formation of Li2O, an almost unlimited variation of new LIB anodes, offering tailored specific capacities and operational potentials, appears conceivable. Following these considerations, we finally discuss the potential advantages of CAMs compared to pure conversion and alloying materials and provide some general principles for the design of new members of the conversion/alloying class, which will ideally inspire scientists to develop further enhanced anodes for next generation lithium-based batteries.
TMxOy + 2yLi+ + 2ye− ↔ xTM0 + yLi2O | (1) |
Due to their highly promising specific and, in particular, volumetric capacities – frequently up to two to six times higher than that of the state-of-the-art graphite anode (depending on the molar mass and oxidation state of the TM as well as the density of the oxide) – tremendous research efforts were undertaken since then to improve the understanding of this reaction mechanism and enhance the electrochemical properties of the conversion-type anodes. Moreover, it was found that such reversible displacement-type reactions occur likewise for a variety of other first-, second-, and third-row transition-metal-based oxides, including Mn,60 Cr,61 Mo,62 Ru,63 or W,64 as well as the corresponding transition metal sulfides,65 nitrides,66 fluorides,67 phosphides,68 hydrides,69 or carbonates.70–72 All these compounds are commonly referred to nowadays as ‘conversion materials’ and an extensive review on this new materials' class was presented in 2010 by prolific and experienced researchers in this field.41
Herein, we will thus focus in particular on those studies reported after 2010 providing an enhanced understanding of the reaction mechanism, mainly carried out on transition metal oxides and fluorides. We may briefly note at this point that transition metal fluorides are considered as alternative high energy LIB cathodes rather than anodes due their relatively higher lithium reaction potential (>2 V), originating from the pronounced ionic character of the TM–F bond.41 Nevertheless, it is commonly agreed on that the insights obtained for fluorides are applicable also for conversion materials in general. As a matter of fact, it was shown for both transition metal oxides59,73 and fluorides,51,74 that upon lithiation, i.e., reduction, an interconnected, percolating network of metallic nanograins, having a size of about 1–3 nm, is formed. This metallic nano-network apparently serves as electron conduction pathway throughout the initial primary particle, thus ensuring the local availability of electrons for the reversible formation of the surrounding quasi-amorphous/nano-crystalline lithium oxide/fluoride. Consequently, the diffusivity of the transition metal cations is of great importance, as it determines the size of these metallic nanograins and, as a result, the realization of a continuous electron-conducting network.51 This consideration was confirmed by comparing the lithiation of iron and copper fluoride – the latter, indeed, revealing significantly larger, isolated metallic nanograins (5–12 nm), due to the higher mobility of copper cations, and a lack of reversibility for the LiF formation.51 Detailed TEM investigations of iron fluoride52 and nickel oxide53 nanoparticles revealed, furthermore, comprehensive insights into the lithiation mechanism itself. While the mobility of lithium ions is very fast at the particle surface52 and/or along the interface of initially already present as well as upon lithiation formed nanodomains,53 its penetration into the bulk particle is relatively much slower,52 suggesting that the conversion reaction takes place layer-by-layer,52 preferably at the aforementioned interfaces,75 which may result in a heterogeneous reaction front in larger particles.53 Accordingly, the nano-crystalline (2–7 nm)53 nature of the once lithiated particles,39,59,74,76,77 indicated also by the subsequently sloped potential profile,78 appears highly beneficial with respect to high-power application for such materials. In fact, even though the initial penetration into the bulk may be relatively slow, a particle with a size of about 10 nm can be fully lithiated within a few minutes, indicating that (dis-)charge rates of up to 20 C may be applied without substantial capacity loss,52 which is in good agreement with earlier findings by Tarascon and co-workers for iron oxide nanoparticles grown directly on a nanostructured copper current collector.45 These findings, however, challenge the initial suggestion39,79 that the greatest issue of conversion-type materials towards their practical use in lithium-ion batteries, namely, the extensive voltage hysteresis between charge and discharge, resulting in a relatively low energy storage efficiency and internal heat evolution, thus, adding also a safety issue, originates from simple kinetics, related to the substantial structural rearrangement upon (de-)lithiation and the insulating nature of most conversion-type materials.80,81 Indeed, neither cycling conversion-type electrodes at elevated temperature (i.e., 100 °C),82 nor the smart design of advanced nanostructures,45,83 the incorporation of highly conductive carbonaceous nanomaterials,84,85 the introduction of metallic dopants,86 or the partial replacement of one transition metal by another to enhance the electronic conductivity87 resulted in a significant decrease of the voltage hysteresis; though certainly enhancing the electrochemical performance in terms of cycling stability and rate capability.88 Instead, based on density functional theory (DFT) calculations for FeF3, Ceder and co-workers proposed that this voltage hysteresis may be related to different reaction paths for the lithiation and delithiation process, originating from the relatively higher mobility of lithium compared to iron ions, thus being intrinsic to the specific material and to its structure.89
The extensive study of nano-sized conversion electrodes, however, has emphasized another great challenge for the implementation of conversion-type materials in practical LIBs: the catalytically activated,90,91 continuous electrolyte decomposition, augmented by the increased surface area and detrimentally affecting their cycling stability.39,77 As this electrolyte decomposition appears to be partially reversible, it can be easily tracked by a capacity increase at low potentials upon lithiation (ca. <0.8 V) and high potentials upon delithiation (ca. >2.0 V).92–94 This partial reversibility was initially assigned to the dissolution of organic, i.e., oligomeric or polymeric (CH2–CH2O)n-like electrolyte decomposition products.58,95 A rather recent combined ex situ XPS and atomic force microscopy (AFM) study on CuO thin films, on the contrary, indicates that the partial reversibility may be ascribed to the dissolution of (large) Li2CO3 particles, which are formed at the electrode surface upon lithiation and are dissolved when the electrode is delithiated.96 Considering the above mentioned reversible conversion reaction of transition metal carbonates,70,71 and particularly of Li2CO3 in presence of the freshly formed transition metal nanograins,72 this explanation appears conceivable.
This brief and certainly not exhaustive summary of recent findings on conversion-type materials illustrates that the most challenging issues towards their implementation in commercial cells still remain to be solved. But there has been also substantial progress. For instance, the issue of continuous electrolyte decomposition may be very well addressed by using internally nanostructured materials, i.e., relatively larger particles built up of nanodomains, thus providing an extensive network of interfaces for fast lithium transfer.97 Another approach may be the passivation of the particle surface with protective coatings as, for instance, carbon77,98 – an already very well established procedure for lithium-ion electrode materials – resulting in a stabilized solid electrolyte interphase (SEI) and preventing an intimate contact between the conversion material particles and the electrolyte. Also, as reported by Oumellal et al.,69 the voltage hysteresis appears to be highly dependent on the nature of the anion, decreasing along the following trend: fluorides > oxides > sulfides > nitrides > phosphides > hydrides. Interestingly, this trend is in rather good agreement with Pearson's concept of hard and soft acids and bases,99,100 giving hope that further progress can be achieved to enhance the energy storage efficiency and reduce the (de-)lithiation potential range of conversion-type electrodes.
And not least, several recent lithium-ion full-cell studies employing conversion anodes have shown very promising performance, as nicely reviewed by Aravindan et al.101 Especially, a composite of porous carbon and Fe3O4 combined with a Li[Ni0.59Co0.16Mn0.25]O2-based cathode102 as well as a composite of perforated graphene and Fe3O4 combined with a LiMn2O4103 cathode revealed exceptional cycling stabilities with a capacity retention of about 64% after 1000 cycles and around 66% after 10000 cycles, respectively. It should be noted, though, that this outstanding performance was achieved by pre-lithiating both anode composites in order to bypass the initial charge loss, which would result in a significant decrease in energy density due to the need of an oversized cathode.104 To be noticed, however, that a high initial charge loss is not only an issue related to conversion-type materials and recently emerged strategies, like the addition of sacrificial lithium salts to the cathode electrode composite,105 the utilization of stabilized lithium powders,106 or the implementation of an internal lithium source107 hold the great promise that it may be overcome soon.
One approach, to address these two issues is the partial replacement of the electrochemically inactive transition metal by an element which can reversibly form an alloy with lithium. In addition to the thus increased number of lithium ions hosted per formula unit, such alloying materials commonly react with lithium at significantly lower voltages,30,31,34 potentially resulting in a decreased overall reaction voltage. As these substances accordingly combine both lithium storage mechanisms in one single compound, we will refer to them herein in the following as ‘conversion/alloying materials’, CAMs. In this case, the initially formed Li2O matrix, supported by the electron conducting TM0 nano-network, has also a buffering effect for the alloying reaction, thus, beneficially combining the advantages of the two reaction mechanisms (Fig. 3). While showing some similarities with the concept of intermetallic alloying compounds,109,110 the combination of conversion and alloying in a single compound provides the additional benefits of a very fine distribution of the alloying element (initially on the atomic scale) and the substantially lower volume expansion (40–100%)52,59,76 and particle morphology evolution53,111–113 of conversion materials. Nonetheless, as both mechanisms are characterized by significant volume variations and structural rearrangement, the final lithium storage properties and the reversibility of the (de-)lithiation reaction remain highly dependent on the particle and electrode architecture. It is thus not surprising that the advent of nanotechnology and the continuously increasing interest in such materials has recently led to great improvements regarding their electrochemical performance.
Fig. 3 Illustrative summary of the beneficial combination of conversion- and alloying-type lithium storage in conversion/alloying materials (CAMs). |
ZnTM2O4 + 8Li+ + 8e− → 2TM0 + 4Li2O + Zn0 | (2) |
Zn0 + Li+ + e− → LiZn | (3) |
And among the three ‘classic’ representatives of this concept – ZnFe2O4 (ZFO), ZnCo2O4 (ZCO), ZnMn2O4 (ZMO) – ZFO is at present the most and, in fact, also the first studied compound. In 1986, Chen et al.115 investigated the possible application of ZFO as active material for lithium-based batteries, assuming that lithium ions may be reversibly inserted into the spinel framework. However, the chemical lithiation was limited to a maximum uptake of 0.5 lithium ions per formula unit of ZFO and did not appear reversible. The achievement of substantially higher and, in particular, reversible lithium storage was then reported in 2004 by NuLi et al.,116 studying the electrochemical lithiation of nanocrystalline ZFO thin films. They observed reversible specific capacities of around 560 mA h g−1, i.e., about half of the theoretical value of 1000.5 mA h g−1 (see Table 1). Subsequent studies of Sharma et al.117 on submicron-sized particles in 2008 and Guo et al.118 on hollow spherical particles in 2010 revealed further increased capacities up to about 900 mA h g−1, i.e., approaching the theoretical maximum value. In addition, Guo et al.118 performed an ex situ selected area electron diffraction (SAED) characterization and confirmed the earlier proposed117 formation of Li–Zn and metallic iron in the fully lithiated state and ZnO and Fe2O3 in the fully delithiated state, which was later on affirmed inter alia by Xing et al.119
2Fe0 + 4Li2O + LiZn ↔ ZnO + Fe2O3 + 9Li+ + 9e− | (4) |
Theoretical Li uptake per formula unit [#] | Theoretical specific capacity/mA h g−1 | Contribution of the alloying reaction [%] | Average delithiation voltage(s)/V vs. Li/Li+ | |
---|---|---|---|---|
a According to Teh et al.249 the delithiation voltages of 1.5–1.65 V are ascribed to the re-oxidation of Zn0. b Assigned to the dealloying of initially formed metallic tin. c These values were determined as the delithiation voltage at which half of the hosted lithium was released. Generally, it should be noted that the here given values are dependent on the applied current or sweep rate as well as the internal resistance of the cell and may thus be considered more as a coarse guideline. Also, please note that the TM2SnO4- and TMSnO3-type stannates commonly show a rather sloped potential profile between the two given values, which may be characterized by additional features related to the re-oxidation of tin, though apparent frequently only in the corresponding CV curves. | ||||
ZnFe2O4 | 9 | 1000.5 | 11.1 | 1.5a/1.75 |
ZnCo2O4 | 975.5 | 1.65a/2.1 | ||
ZnMn2O4 | 1008.1 | 1.2/1.6a | ||
Co2SnO4 | 12.4 | 1105.7 | 35.5 | 0.55b/2.0 |
Mn2SnO4 | 1135.9 | 0.55b/1.25 | ||
Ni2SnO4 | 1107.5 | 0.56b/2.1 | ||
CoSnO3 | 10.4 | 1235.3 | 42.3 | 0.6b/2.1 |
NiSnO3 | 1236.6 | 0.55b/2.2 | ||
Zn0.9Fe0.1O | 2.9 | 966.1 | 31.0 | 1.3c |
Zn0.9Co0.1O | 962.4 | 1.4c | ||
Sn0.9Fe0.1O2 | 7.96 | 1477.2 | 49.7 | 0.5b/1.5 |
Co2SiO4 | 4 | 510.6 | 0 | ∼2.0 |
Fe2SiO4 | 526.1 | ∼2.0 | ||
Mn2SiO4 | 530.8 | ∼1.4 |
Similarly, it was reported that ZnCo2O4 (ZCO), firstly reported as lithium-ion anode in 2004 as well,120 and ZnMn2O4 (ZMO), reported for the first time in 2008,121 reversibly form zinc oxide and cobalt oxide (4), respectively, manganese oxide (5) after the first lithiation. Nevertheless, in both cases, the detailed reaction mechanism is not fully clarified, yet. Some studies detected only Co3O4122,123/Mn3O4124 in the delithiated state, while others identified a mixture of the respective monoxide and the spinel-structured oxide, i.e., CoO/Co3O4125 and MnO/Mn3O4.126 It is apparently challenging to stoichiometrically balance the following two eqn (5) and (6) without assuming either an additional oxygen source or oxygen-deficient metal oxide phases. Since this question remains to be properly addressed, we may simply refer herein to an oxide phase with an undetermined TM:O ratio, including also the possible presence of different transition metal oxide phases:
2Co0 + 4Li2O + LiZn ↔ ZnO + CoxOy + 9Li+ + 9e− | (5) |
2Mn0 + 4Li2O + LiZn ↔ ZnO + MnxOy + 9Li+ + 9e− | (6) |
Accordingly, we may thus express the general reversible (de-)lithiation reaction for spinel-structured ZnTM2O4 as follows:
2TM0 + 4Li2O + LiZn ↔ ZnO + TMxOy + 9Li+ + 9e− | (7) |
Apart from such fundamentally important studies, targeting an enhanced understanding of the detailed reaction mechanism, the majority of the published research activities focused on the improvement of the electrochemical performance when used as lithium-ion anode. For this purpose, basically four approaches have been (partially simultaneously) followed for this materials' class (Fig. 4; selected examples are also presented in Table 2):
Approach(es) | Example | Specific capacity/mA h g−1 | Specific current/A g−1 | Long-term cycling capability | Electrode composition [active material:conductive carbon:binder]/wt% | Ref. |
---|---|---|---|---|---|---|
(1) | Hollow ZnCo2O4 dodecahedra | 886 | 4.5 | 990 mA h g−1 after 50 cycles at 0.1 A g−1 | 80:10:10 | 141 |
692 | 7.2 | |||||
575 | 9.0 | |||||
Mesoporous ZnCo2O4 twin microspheres | 1005 | 2.0 | 550 mA h g−1 after 2000 cycles at 5.0 A g−1 | 70:20:10 | 134 | |
920 | 5.0 | |||||
790 | 10.0 | |||||
Porous ZnMn2O4 microspheres | 896 | 0.2 | 800 mA h g−1 after 300 cycles at 0.5 A g−1 | 75:15:10 | 128 | |
654 | 0.5 | |||||
496 | 1.0 | |||||
(1) & (2) | Mesoporous ZnFe2O4 submicro-spheres/graphene composite | 940 | 0.5 | 970/770 mA h g−1 after 200/500 cycles at 0.5/1.0 A g−1 |
70*:20:10
(*incl. 22% graphene) |
148 |
820 | 1.0 | |||||
650 | 2.0 | |||||
(2) | ZnMn2O4/graphene nanosheets | 806 | 0.2 | 650 mA h g−1 after 1500 cycles at 2.0 A g−1 |
90*:—:10
(*incl. 22.5% graphene) |
149 |
568 | 3.2 | |||||
Co2SnO4/graphene nanocomposite | 1103 | 0.1 | 1046 mA h g−1 after 100 cycles at 0.1 A g−1 |
50*:30:20
(*incl. 9% graphene) |
180 | |
910 | 0.25 | |||||
797 | 0.5 | |||||
710 | 1.0 | |||||
CoSnO3/graphene nanocomposite | 957 | 0.1 | 724 mA h g−1 after 50 cycles at 0.05/0.2 A g−1 for discharge/charge |
75*:10:15
(*incl. 19% graphene) |
190 | |
829 | 0.2 | |||||
706 | 0.4 | |||||
539 | 0.8 | |||||
(1) & (3) | Porous, hollow ZnFe2O4/ZnO/C octahedra | 934 | 1.0 | 988 mA h g−1 after 100 cycles at 2.0 A g−1 |
70*:15:15
(*incl. 20% carbon & 22% ZnO) |
139 |
887 | 2.0 | |||||
842 | 5.0 | |||||
762 | 10.0 | |||||
Porous Mn2SnO4/Sn/C cubes | 971 | 0.5 | 908 mA h g−1 after 100 cycles at 0.5 A g−1 |
80*:10:10
(*incl. 13% carbon) |
185 | |
775 | 1.0 | |||||
550 | 2.0 | |||||
(2) & (3) | Carbon-coated ZnFe2O4 nanoparticles/graphene composite | 585 | 0.93 | 705 mA h g−1 after 180 cycles at 0.23 A g−1 |
80*:10:10
(*incl. 38% carbon & graphene) |
150 |
505 | 1.86 | |||||
404 | 4.64 | |||||
(3) | Carbon-coated ZnFe2O4 nanowires153/nanoparticles154,155 | 997153 | 0.1153 | 1091 mA h g−1 after 190 cycles at 0.1 A g−1155 |
80*:10:10153
(*incl.2.5%carbon) 75*:20:5154 (*incl. 13% carbon) 80*:10:10155 (*incl. 14% carbon) |
153–155 |
862153 | 1.6153 | |||||
750153 | 3.2153 | |||||
310154 | 7.8154 | |||||
216155 | 20.0155 | |||||
Carbon-coated Zn0.9Fe0.1O nanoparticles | 720 | 0.1 | 800 mA h g−1 after 30 cycles at 0.05 A g−1 |
75*:20:5
(*incl. 18.5% carbon) |
46 | |
670 | 0.19 | |||||
580 | 0.48 | |||||
475 | 0.95 | |||||
360 | 1.9 | |||||
Carbon-coated Sn0.9Fe0.1O2 nanoparticles |
1726/1360*
(*incl. the carbon) |
0.05 | 1519/1195* mA h g−1 after 10 cycles at 0.05 A g−1 |
75*:20:5
(*incl. 21% carbon) |
50 | |
(4) | Mn-doped ZnFe2O4 nanoparticles | 900 | 0.5 | ∼100% capacity retention after 50 cycles at 0.1 A g−1 | 50:30:20 | 164 |
790 | 1.0 |
(1) Synthesis and design of (meso-)porous micro-sized secondary particles, built up out of very fine nanocrystals, for which the porous structure may buffer the occurring volume variations while the nanosize of the primary particles allows for short lithium and electron transport pathways123,125,127–130 (Fig. 4a).
This approach has particularly provided an excellent impression of the steadily improving ability of scientists to develop smartly designed new material architectures using advanced synthesis techniques, including the realization of (hollow) submicro- and micro-spheres,131–134 “yolk–shell” or “ball-in-ball” structures,135–137 (sub-)micro-cubes,134,138 and (hollow) porous micro-polyhedra.139–141 And apart from the frequently rather complex synthesis procedures, these compounds commonly show enhanced specific capacities, cycling stability, and rate capability. Mesoporous ZCO twin microspheres,134 for instance, revealed an impressive high rate performance, offering specific capacities of 920 and 790 mA h g−1 for applied currents of 5.0 and 10.0 A g−1, respectively, and a capacity retention of 550 mA h g−1 after 2000 cycles at 5.0 A g−1. Similarly, superior lithium storage capability was reported for porous (core–shell)137 ZMO microspheres, presenting a stable capacity of 800 mA h g−1 for more than 300 cycles128 and capacities of about 610, 530, and 460 mA h g−1 for specific currents of 1.0, 2.0, and 4.0 A g−1, respectively.137 When investigating the impact of the particle morphology and shape, it was moreover found that octahedrally-shaped ZFO particles appear advantageous concerning their electrochemical properties as anodes,119,142 which Zhong et al.142 attributed to the relatively larger fraction of {111} surface planes, though this is not completely understood, yet. Despite these great advantages of mesoporous, highly nanocrystalline structures, however, their intrinsically large surface area also leads to relatively lower coulombic efficiencies due to the aforementioned high reactivity of transition metals towards state-of-the-art organic electrolytes.90,91
(2) Introduction of carbonaceous particles and nanostructures to enhance the electronic conductivity within the electrode and to avoid the loose of contact upon continuous (de-)lithiation by forming strong interactions between the carbonaceous heterostructures and the active material nanoparticles (Fig. 4b).
The incorporation of carbonaceous particles and nanostructures as, for instance, graphite flakes,143 carbon nanotubes (CNTs),144 3-dimensional (3D) porous carbons,145,146 or graphene147–151 mainly targets the realization of percolating electron conducting networks within the final electrode composition and enables the utilization of very fine nanoparticles (frequently <20 nm) without facing severe particle agglomeration upon electrode preparation and/or cycling. Accordingly, such active material/carbon composites commonly show improved cycling stability and rate capability. Very impressive results were inter alia reported by Xiong et al.149 for a chemically integrated ZMO/graphene composite, achieving an outstanding cycling stability of 1500 cycles with a specific capacity of more than 600 mA h g−1 at 2.0 A g−1 and still 570 mA h g−1 for a further increased specific current of 3.2 A g−1. Nevertheless, depending on the amount of carbonaceous additives, this approach may lead to decreased gravimetric capacities and, more importantly, the issue of the insufficiently stable ZnTM2O4/electrolyte interface remains to be solved.
(3) Application of polymeric or carbonaceous coatings to buffer occurring volume changes, enhance the electronic conductivity, and passivate the highly reactive ZnTM2O4 (nano-)particle surface in order to stabilize the electrode/electrolyte interface and prevent continuous electrolyte decomposition (Fig. 4c).
While the application of polymer particle coatings like polyaniline132 or polypyrrole152 showed some promising results and is of particular interest for lithium-ion polymer batteries, the best electrochemical performances following this approach were so far obtained for carbon-coated ZnTM2O4 nanostructures. Carbon-coated ZFO, for instance, revealed excellent high-rate capability, providing specific capacities of 750 mA h g−1 at 3.2 A g−1,153 525 mA h g−1 at 3.9 A g−1,154 400 mA h g−1 at 4.6 A g−1,150 310 mA h g−1 at 7.8 A g−1,154 and still 216 mA h g−1 at a specific current as high as 20.0 A g−1,155 which was assigned to the decreased charge transfer resistance, originating from the enhanced electronic conductivity.156,157 The improved properties may, however, in part also be related to the substantially stabilized electrode/electrolyte interface158–161 and inhibited ion dissolution,155 as a direct contact between the ZnTM2O4 particles and the electrolyte is prevented. In fact, it is well known that carbonaceous anodes generally form very stable solid electrolyte interphases.162 This is also true for carbon-coated ZFO, as very recently confirmed by in situ Raman spectroscopy160 and ex situ X-ray absorption spectroscopy (XAS).159,161 Another at least as important finding was reported by Kim et al.,153 who observed that the voltage hysteresis, discussed in detail in Section 2, may be, indeed, substantially reduced for carbon-coated ZFO to about 0.5 V, if the kinetic contribution may be further reduced by designing advanced electrode architectures, e.g., by combining carbonaceous coatings and graphene-based heterostructures.150 As a matter of fact, these results render the application of carbonaceous coatings most promising for the development of advanced lithium-ion conversion/alloying-type anodes.
(4) Doping with transition metal cations, providing different valence states to increase the electronic conductivity of the CAM itself (Fig. 4d).
This approach has been studied only very rarely so far. In fact, the partial substitution of zinc by nickel resulted in an inferior capacity retention for nanocrystalline ZFO.163 Contrarily, the incorporation of manganese164 instead of zinc lead to enhanced specific capacities, cycling stability, and also rate capability, which the authors assigned to a reduced charge transfer resistance.164 Apparently, however, further comprehensive studies will be required to investigate the potential beneficial impact of transition metal doping before any conclusion can be drawn.
Fig. 5 Lithium-ion full-cell studies employing ZnTM2O4-based anodes: (a) Schematic illustration of ZnCo2O4 nanowire arrays deposited on carbon cloth, serving simultaneously as substrate and current collector, as reported by Liu et al.165 and (b) its electrochemical performance, when coupled with a LiCoO2-based cathode (2.2–3.7 V, 0.2 A g−1). (c) Schematic illustration of the ZnMn2O4–graphene nanocomposite reported by Xiong et al.151 and (d) its rate performance when combined with 2D LFP nanosheets as cathode (blue spheres; 0.9–3.9 V) in comparison with a standard lithium-ion full-cell including graphite and commercial LFP (red spheres). (e–h) Electrochemical performance of carbon-coated ZnFe2O4 nanoparticles in lithium-ion full-cell configuration vs. a LFP-C/CNT-based cathode as reported by Varzi et al.166 (e) Schematic illustration of the prototype, (f) capacity vs. cycle number plot (4.2–2.8 and 2.8–0.05 V for the cathode and anode, respectively), (g) the corresponding Ragone-like plot (calculated on the base of the total active material content), and (h) the development of the ZnFe2O4-C anode (in brown) and LFP-C/CNT cathode (in black) voltage profiles when varying the degree of pre-full-cell-cycling lithium doping from fully delithiated (left) over 200 mA h g−1 (middle) to 600 mA h g−1 (right) at different current densities. |
Fig. 6 Schematic illustration of the impact of the comprised transition metal on the theoretical specific capacity and the (de-)lithiation voltage of ZnTM2O4 CAMs. |
TMxSnOy + (2y + 4.4)Li+ + (2y + 4.4)e− → xTM0 + Li4.4Sn + yLi2O (with x = 1 or 2; y = 3 or 4, depending on x) | (8) |
xTM0 + Li4.4Sn + yLi2O ↔ xTMO + SnO2 + (2y + 4.4)Li+ + (2y + 4.4)e− | (9) |
In fact, the first studies on TM2SnO4 (TM = Co, Mn) in 2001182 and 2002175 introduced electrochemically inactive TMs into tin oxides as spectator ions to allow an X-ray-based correlation of the structural evolution to the occurring electrochemical processes for these commonly upon lithiation amorphized materials. Nevertheless, presumably related to the relatively low anodic cut-off potential of 2.0 V, they did not observe any direct evidence for a re-oxidation of the initially reduced metals.175 This finding was challenged in 2003 by Huang et al.,186 who proposed the reversible formation of CoO for a higher anodic cut-off potential of 3.0 V to explain at least part of the very high capacity, they obtained for CoSnO3 (∼1200 mA h g−1). Further enhancing the understanding of the (de-)lithiation mechanism, Alcántara et al.176 found that also tin is re-oxidized from Sn0 to Sn4+ upon delithiation of Co2SnO4. The overall reaction mechanism, as given in (8) and (9), was then finally unraveled by performing ex situ high-resolution transmission electron microscopy (HRTEM) on (de-)lithiated NiSnO3.192 Concerning their utilization as lithium-ion anodes, however, these materials commonly suffer rapid capacity fading, which was assigned to the extensive volume changes and structural reorganization upon lithium uptake and release. As a matter of fact, the advantageous lithium storage capability of Sn compared to Zn is accompanied by a disadvantageous augmented volume variation.181 It is interesting to note that, as a result, initially the conversion reaction (occurring at relatively higher voltages) becomes less reversible, later on followed by the alloying reaction.180,190 This observation may be interpreted by a more pronounced sensitivity of the conversion reaction towards dramatic volume changes, which assumedly lead to the breakdown of the continuous, percolating electron conducting network.
To overcome these challenges, scientists followed mainly the same approaches as discussed for Zn-based CAMs (see examples presented in Table 2), including inter alia the incorporation of CNTs178 or graphene.180,190 For a Co2SnO4/graphene composite,180 for instance, capacities of 800, 710, and 510 mA h g−1 were obtained when applying a specific current of 0.25, 0.5, and 1.0 A g−1, respectively. Moreover, the authors reported a stable capacity of about 1090 mA h g−1 after 100 cycles at 0.1 A g−1, assigning this very good performance to the enhanced structural integrity of the electrode composite, an increased electronic conductivity, and the reduced charge transfer resistance compared to the pure active material. Likewise, the application of carbonaceous surface coatings revealed substantial improvement,177,179,185,187–189 in particular for coating thicknesses of 5–10 nm177 and more,188 as relatively thinner coatings (2–3 nm) did not provide a sufficient mechanic stability to buffer the extensive volume variations.177 This approach did not only result in remarkable capacities of around 500 and 364 mA h g−1 for currents of 1.0 and 2.0 A g−1, respectively,188 but also in a stable capacity for 400 charge/discharge cycles.187
Nonetheless, despite the larger theoretical capacities and decreased (de-)lithiation potentials,175 the successful realization of long-term stable, high-performing lithium-ion anodes based on tin-comprising CAMs certainly makes even greater demands on scientists to develop advanced electrode architectures compared to Zn-comprising ones and research activities are currently still at an early stage.
The initial idea of utilizing alloying PTMOs, like SnO2 or ZnO, rather than the pure metals was based on the concept that the in situ formed Li2O (10) would serve as an electrochemically inactive matrix, buffering the occurring volume variations for the subsequent (de-)alloying reaction (11) and acting as a glue for the thus formed PTM nanoparticles.35–38 Nonetheless, the concomitant dramatic charge loss upon first lithiation (∼48% for SnO2 and ∼67% for ZnO) and, even more important, the fact that this matrix eventually does not prevent a continuous aggregation of metallic particles upon cycling,37,193–199 resulting in a continuous capacity decay, has up to now precluded their application in commercial batteries.
PTMOx + 2xLi+ + 2xe− → PTM0 + xLi2O | (10) |
PTM0 + yLi+ + ye− ↔ LiyPTM (with ‘y’ depending on the PTM) | (11) |
Nonetheless, both zinc oxide and particularly tin oxide200 were studied extensively within the past years with respect to their reversible lithium storage capability. Remarkably, some of these studies201–208 reported initial capacities exceeding the theoretical maxima for the alloying reaction only (329 mA h g−1 for ZnO and 782 mA h g−1 for SnO2) when charging (delithiating) to sufficiently high anodic cut-off potentials. Considering the findings of Poizot et al.39 and the suggestion of Courtney and Dahn,37 that the Li2O matrix may get decomposed above 1.3 V, it was proposed that also in these cases the partial re-oxidation of the PTM may be possible.201–203 In fact, ex/in situ X-ray diffraction (XRD),208–210 XPS,208,211 and X-ray absorption spectroscopy (XAS)197 studies later on confirmed the partial re-oxidation of PTM to PTMOx, though only for the first cycle(s). With regard to the findings for pure conversion materials, reviewed in Section 2, it appears that the high diffusivity of the PTM in the Li2O matrix and the resulting emergence of extremely large PTM particles (up to a few hundred nanometer)193–199 inhibit the formation of a continuous percolating electronically conductive network, which is required for the degradation of Li2O. However, it was also shown by in situ TEM that the application of, e.g., carbonaceous coatings can effectively suppress the formation of such large PTM crystals212 and enhance the reversibility of the overall (de-)lithiation and, particularly, the conversion-type reaction (as indicated by the increased capacity at higher voltages).55,56,213,214 As a result, nanoparticulate ZnO confined in a carbonaceous matrix,214 for example, revealed a stable capacity of about 700 mA h g−1 for 200 cycles at 0.1 A g−1. Similarly, ZnO/C nanorods coated with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT–PSS)210 showed a very impressive cycling stability, retaining a specific capacity of around 620 mA h g−1 after 1500 cycles applying a specific current of 0.98 A g−1.
Nevertheless, it appears important to note that especially in case of Sn-based oxides, due to the substantially larger lithiation capacity and consequent volume expansion of tin, the confining coatings have to withstand an enormous mechanical strain, which in turn calls for sufficiently thick, i.e., stable coating layers.212
Zn1−xTMxO + (3 − x)Li+ + (3 − x)e− → (1 − x)LiZn + xTM0 + Li2O | (12) |
Fig. 7 (a) Comparison of galvanostatically (dis-)charged electrodes comprising Co-doped (green), Fe-doped (orange), and pure ZnO (light grey) as active material (1st cycle: 0.024 A g−1, following cycles: 0.048 A g−1; electrode composition (active material:conductive carbon:binder): 75:20:5).46 (b) Representative potential profiles (cycles 2–5) of alloying-type metal oxides (SnO2 in red) and conversion/alloying-type binary metal oxides (carbon-coated Sn0.9Fe0.1O2 in blue; as reported by Mueller et al.50) to depict the direct observation of an enhanced reversibility of the Li2O formation by incorporating a TM (here: Fe) dopant (electrode composition see Table 2; note that the presented capacity for Sn0.9Fe0.1O2-C is based solely on the content of Sn0.9Fe0.1O2). |
Intriguingly, Co-doped ZnO showed subsequently also a very stable cycling performance, while for Fe-doped ZnO the additional application of a carbonaceous coating, simultaneously interconnecting the single nanoparticles, appeared inevitable46 (see also Table 2). Considering that both TMs commonly enable a very reversible conversion reaction (see Section 2), one possible reason for the diverging electrochemical performance may be the different oxidation state of the TM dopant – Fe3+vs. Co2+ – as very recently revealed by Giuli et al.47 who performed an in-depth structural characterization of these compounds. However, further studies will be required to affirm or negate a potential impact of the TM oxidation state on the electrochemical performance. Generally, the advantageous impact of the TM dopant, confirmed also by Yue et al.,49 was assigned to an enhanced nanocrystallinity, i.e., the inhibition of extensive Zn crystal growth, in combination with a promoted LiZn alloying reaction as revealed by a comparative in situ XRD analysis of pristine and TM-doped ZnO.46,48 In addition, the authors proposed that the electrochemically inactive TM nano-network ensures the required electron supply for the reduction of Li2O throughout the primary particle.46,48 In fact, there is a strong intercorrelation between these effects, as recently reported by Su et al.198 They observed that the Zn crystal growth and the LiZn alloying reaction are current-dependent competitive processes. Accordingly, one may assume that – in addition to a constricting effect215 – the percolating conductive TM network results in a relatively reduced current density, thus, kinetically favoring the alloying reaction rather than the zinc aggregation.
The general applicability of this approach was very recently corroborated by Mueller et al.50 who performed an investigation on the influence of Fe-doping for SnO2-based lithium-ion anodes. Indeed, a direct comparison of the potential profiles recorded for pure SnO2 and Fe-doped SnO2 (Fig. 7b) nicely illustrates the extended capacity gain at higher potentials, assigned to the deformation of Li2O;55 similarly, in fact, to an early study of Chen et al.216 on iron oxide nanoparticles encased in hollow, submicron-sized SnO2 spheres. Furthermore, capacity-limited cycling revealed also a beneficial impact of the Fe dopant on the reversibility of the alloying reaction, as reflected by a very stable performance at 600 mA h g−1 with high coulombic efficiency, similarly to a previous study on Mo-doped SnO2.217 Nevertheless, when (dis-)charging the electrodes in an extended voltage range (0.01 to 3.0 V), the (substantially higher) capacity gradually decreased, suggesting that further improvement by, for instance, designing advanced electrode architectures218 will have to be realized; similarly as for the Sn-based CAMs reviewed in Section 3.5, in fact.
TM2SiO4 + 4Li+ + 4e− ↔ 2TM0 + Li4SiO4 | (13) |
This finding is scientifically very interesting, as the formation of Li4SiO4 – in fact, a very good lithium ion conductor228 – starting from silica was considered to be irreversible.229,230 However, the theoretical capacity resulting from this reaction is rather limited (510 mA h g−1 for TM = Co, see also Table 1) and obtained along an entirely sloped potential profile,226 rendering these materials at present unsuitable for high-energy lithium-ion batteries.
In 2005, SONY successfully introduced a Sn–Co–C composite (presumably having a ratio of about 30:30:40), within which the cobalt/carbon matrix dilutes the occurring volume variation and prevents tin aggregation and crystallization upon de-/lithiation.30,231–234 Interestingly, the role of cobalt appeared unique for the latter purpose, as revealed by extensive combinatorial studies including a variety of transition metals.30,231–235 As a matter of fact, the same effect, that is the suppression of crystallite growth for the alloying element, is observed for the incorporation of a TM dopant (Fe or Co) in ZnO, i.e., Zn-based CAMs with an increased alloying contribution (Fig. 8); nevertheless, advantageously not limited to costly and toxic cobalt and for substantially lower TM contents (e.g., Zn:TM = 10:1).46,48 With respect to the findings of Su et al.,198 stating that the aggregation and alloying reaction would be two competing processes, the authors proposed that the presence of the metallic TM nano-network may, on the one hand, lead to an enhanced electronic conductivity throughout the initial primary particle and thus kinetically favor the alloying reaction and, on the other hand, physically hinder the diffusion of the alloying element nanocrystals48 (see Section 4.2).
Fig. 8 Schematic illustration of the impact of introducing a TM dopant into ZnO on the lithiation mechanism and LiZn (Zn0) crystallite growth according to the findings of Mueller et al.48 |
Beside the Sn–Co–C composite, also SiOx (x ≈ 1) is nowadays commercially employed as alloying-type active material, though only added in small percentage to graphite-based anodes.15,30,236 The utilization of amorphous and under-stoichiometric silica, considered to be a dispersion of silicon grains in a SiO2 matrix,30,236 rather than silicon, follows mainly economic considerations, since SiOx can be readily synthesized at large scale.15,236 Additionally, the silica matrix, which is transformed to lithium silicate after the first lithiation,30,229 serves as diluent for the occurring volume variations and as lithium ion conductor.30,228 The associated irreversible lithium consumption, however, is the major issue for the addition of larger percentages of SiOx.15,30 Remarkably, the most successful approach to decrease this initial irreversibility was reported by NEC researchers237 who introduced metallic iron, nickel, or titanium into SiOx thin film electrodes, thus, enhancing the first cycle coulombic efficiency from about 50% to more than 85%. According to an ex situ XPS analysis and the slightly sloped shape of the recorded potential profiles at higher voltages, this improved coulombic efficiency originated from the partially reversible formation of lithium oxide and/or lithium silicate. Notably, when further introducing metallic lithium, the authors were able to achieve first cycle coulombic efficiencies of about 100% and very stable cycling of lithium-ion full-cells. In fact, in the light of the herein reviewed CAMs and the underlying reaction mechanisms, these findings regarding the enhanced coulombic efficiency do not come as a surprise, but nicely highlight the potential advantages of alloying-rich CAMs over pure alloying-type compounds, i.e., enabling the reversibility of the conversion reaction by introducing a non-alloying element, as also observed for TM-doped ZnO and SnO2 (see Section 4.2).
The latter approach may, indeed, eventually help also to overcome the second main challenge, the relatively higher voltage of the conversion reaction, limiting its accessibility in practical lithium-ion full-cells. The implementation of suitable transition metals, re-oxidizing at relatively low potentials (e.g., Mn or Cr), may provide a suitable way in this regard. However, this will have to be accompanied by an enhanced understanding of the interplay of the transition metal redox process and that of the lithiated species. Considering the findings reviewed and discussed herein, it appears favorable to ensure a highly homogeneous distribution of these two compounds in the lithiated state, ideally characterized by ultrafine grain-sizes and highly percolating networks, while at the same time providing a sufficiently high mechanical stability to prevent the crystallite growth of the alloying element. The realization and, in fact, also the investigation of this task is certainly very challenging, but the rapid development in recent years of advanced characterization techniques such as in situ TEM and XAS gives rise to optimism that overcoming these challenges is possible.
The third and last major challenge is not directly related to the materials itself, but to its impact on the electrochemical stability of the electrolyte. In fact, the success of the state-of-the-art anode material graphite is a result of the finding that ethylene carbonate forms a stable SEI on its surface, thus preventing a continuous electrolyte degradation.240 As discussed earlier, the application of carbonaceous coatings targets in this direction and promising results were reported. Nevertheless, the development of an in-depth comprehension of the impact of the chemical nature of the electrode, the exposed crystallite facets, and the composition of the electrolyte may help to address this issue from a fundamental perspective – though the still remaining questions concerning the SEI on graphite render such a systematic investigation a more than extensive research project. Potentially – just like in the case of graphite – the development of new electrolyte systems will eventually enable a stabilized electrode/electrolyte interface.
However, though these composite design strategies are indispensable for realizing high-performance CAM anodes, it appears that particularly the further decrease of the lithium reaction potential and the voltage hysteresis, i.e., the energy density and energy efficiency, respectively, require an optimization of the materials itself, thus, on the atomic scale rather than on the nanoscale. Therefore, an in-depth understanding of the detailed reaction mechanisms upon (de-)lithiation in an electrochemical cell is needed. Nonetheless, based on the insights into pure conversion- and alloying-type compounds, TM-doped PTMOs were developed, showing great promises in this regard, and it is anticipated that further enhancement can be realized by carefully selecting the incorporated TM and PTM cations. The utilization of manganese, for instance, as TM commonly results in relatively low average lithium reaction voltages, as summarized in Table 1.
With respect to the choice of the PTM, zinc certainly has some advantages concerning its availability,174 non-toxicity, and relatively small volume variation upon (de-)lithiation, i.e., volumetric energy density when considering the lithiated state of the anode;30 nevertheless, at the expense of less gravimetric capacity compared to tin. As an alternative, germanium with a theoretical capacity of 1385 mA h g−1 (≙Li15Ge4241), low delithiation voltage of about 0.5 V, and fast lithium diffusion242 would be very promising, in particular, as it has shown a remarkably stable cycling, both in its metallic242,243 as well as in its oxide form,54,244 but its high cost limits its use to niche applications like, for example, space or military applications. Generally, every element, which can reversibly alloy with lithium, appears conceivable to be employed in new CAMs,30–34 but, particularly with respect to its natural abundance, the final target is certainly silicon. In fact, even if not analyzed or discussed by the authors,223 the potential profiles reported for cobalt silicate hollow spheres (Section 4.3) revealed some alloying-like characteristics in the low-voltage region. This observation and the findings of Miyachi et al.,237 regarding the partial reversibility of lithium oxide and/or lithium silicate when introducing metallic iron, nickel, or titanium into SiOx thin film electrodes (see Section 5.2), provide hope that silicon-based CAMs may, indeed, be realized. We may briefly note at this point that the selection of the alloying element should focus not only on the gravimetric capacity but also keeping in mind the volumetric values, comprehensibly summarized and reasoned very recently by Obrovac and Chevrier,30 which are especially important for small-scale LIBs for portable electronic devices.15
Another option to tailor the (de-)lithiation voltage and enhance the energy storage efficiency concerns the variation of the anion. According to Aymard and co-workers,69 the voltage hysteresis decreases in the order fluorides > oxides > sulfides > nitrides > phosphides > hydrides. Moreover, switching from oxides to nitrides or phosphides, for instance, may lead to improved power performances with respect to the high ionic conductivity of the resulting lithium species.245–248
As a result, an almost unlimited number of possible (multi-)cation/(multi-)anion combinations, each characterized by defined lithium reaction voltages and specific capacities appears conceivable.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ee02346k |
This journal is © The Royal Society of Chemistry 2016 |