固体高分子電解質についての考察

Nature Communications volume 14、記事番号: 4884 (2023) この記事を引用

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リチウムイオン電池 (LIB) が商品市場に登場する前は、固体リチウム金属電池 (SSLMB) は、安全性への懸念から 1980 年代後半にほとんど放棄されるまで、有望な高エネルギー電気化学エネルギー貯蔵システムと考えられていました。しかし、開発から 30 年を経て、LIB 技術は現在、ロッキングチェアの化学反応によって課せられるエネルギー量と安全性の限界に近づきつつあります。これらの側面は、学術レベルと産業レベルの両方で SSLMB テクノロジーの研究活動の復活を促しています。この展望記事では、固体高分子電解質 (SPE) について、初期の開発から SSLMB での実装に至るまでの個人的な考察を示し、主要なマイルストーンに焦点を当てます。特に、1990 年代初頭に C. Austen Angell によって提案された結合 SPE と分離 SPE の概念を考慮して SPE の特性について説明します。 SPE の物理化学的および電気化学的特性を改善するための可能な治療法も検討されています。この記事では、理想的な SSLMB を構築する上で欠けているブロックを強調し、将来の充電式高エネルギー電池用の革新的な電解質材料に向けた研究を促進することも目的としています。

1870 年の小説『海底二万里』の中で、ジュール・ヴェルヌは潜水艦ノーチラス号が高度なバッテリーシステムによって動力を供給されていると描写し、ネモ船長は次のように述べています。ジュール・ヴェルヌが提案した高エネルギー電池を構築するという概念は、間違いなく 19 世紀後半では時代を先取りしていましたが、電気の使用によって生み出される驚異に対する当時の人々の関心と一致していました。「すべてに電気を」は、1900 年代初頭に生きた人類の夢でしたが、20 世紀後半、ロッキングチェアの概念に基づいたリチウムイオン電池 (LIB) の発明により現実になります。電気化学エネルギーを貯蔵/伝達するための異なる電位を持つ 2 つのインターカレーションベースの電極)2,3。現在、世界の LIB 生産量は 500 ギガワット時 (GWh) を超える大規模に達しており、600 万台以上の電気自動車 (EV) の電源として機能しています4。 LIB の成功は、「すべてが電気で」という初期の仮説を証明し、エネルギーを消費する人為的活動のより持続可能な開発への新たな道を切り開きます。

LIB の生産能力は過去 10 年間で 10 倍に増加しており5、この需要は主に急成長する EV セクターによって今後 10 ～ 30 年間急速に成長し続けると予想されています4。特に現代の実用的な用途（例えば、道路用および飛行用のEV、ドローン、高度なロボット工学など）によってもたらされる厳しい要件を考慮すると、高性能（エネルギー密度、安全性、コストなど）の充電式バッテリーの必要性も差し迫っています。 .)、固有の安全性、比エネルギー (>500 Wh kg-1) およびエネルギー密度 (>1000 Wh L-1) を含む6。残念なことに、今日の LIB で使用されている非水電解質は、有機カーボネート溶媒 (例: ジメチルカーボネート、エチレンカーボネートなど) の存在により不安定で非常に引火しやすいものです。さらに、比容量が 372 mAh g-1 と比較的低いグラファイト負極も、最先端の LIB の比エネルギーをさらに向上させるための制限要因となっています 6。この点に関して、高エネルギー電極材料（例：リチウム金属（Li°）、リチウム合金、ニッケルリッチな LiNi1-x-yCoxMnyO2（1-x − y > 0.8）など）を組み合わせた固体リチウム金属電池（SSLMB）、硫黄など）と固体電解質を併用することは、現在の LIB 技術の比エネルギー密度の障害を回避するための実行可能なアプローチと考えられています7、8、9。

1 GWh) of SPE-based SSLMBs as power sources for EV and grid storage have been deployed by the Bolloré group since 201013. This is a relevant industrial example of SPE technology capable of providing support for the development of high-performance SSLMBs./p>4 eV for PEO34). The two discs (light gray) on the top and bottom of the SPE membrane represent the blocking electrodes. DC: direct current. c Phase diagram of lithium trifluoromethyl sulfonate (LiCF3SO3)/PEO. The values are taken from ref. 38. The light green and pink areas represent the amorphous phase (abbreviated as AP) region and two-phase region in the PEO-based electrolytes, respectively. PEO(C) and (PEO)3LiCF3SO3(C) denote the crystalline phase of PEO and the salt/polymer complex (i.e., (PEO)3LiCF3SO3), respectively. d Microscopic views of PEO-based SPEs at room (25 °C) and high (>60 °C) temperatures above the melting transition of PEO phases. e Effect of temperature on the ionic conductivity of PEO-based SPEs [(PEO)20LiCF3SO3] and conventional liquid electrolyte solutions (e.g., 1.0 mol kg−1 lithium hexafluorophosphate (LiPF6) per kilogram propylene carbonate). The ionic conductivity values are taken from refs. 39,122./p>4 eV34) for electron jumping between conduction and valence bands (Fig. 2b). Yet, it was not clear whether the transportation of ionic species would be possible at that time. In 1966, Lundberg et al.35 investigated the mixture of metal salts (e.g., potassium iodide) and poly(ethylene oxide) (PEO). They concluded that metal salts interact with PEO and reduce crystallinity. In 1971, M. Armand carried out several ionic conductivity tests with lithium bromide (LiBr)/PEO. From the analysis of the results, he concluded that because of the very high resistance (>1 MΩ) measured at room temperature (ca. 20–30 °C), the utilization of LiBr/PEO for battery applications was not recommended. Two years later, Fenton et al.36 discovered that the mixtures of PEO and low-lattice-energy metal salts (e.g., sodium iodide (NaI), sodium thiocyanate (NaSCN), potassium thiocyanate (KSCN), etc.) become ionically conductive upon warming up the samples (e.g., ionic conductivities for the (PEO)4KSCN complex: 10−7 (40 °C) vs. 10−2 S cm−1 (170 °C)). This key finding rapidly caught the attention of Armand, and he suggested the utilization of these polymeric ionic conductors as solid electrolytes for building solid-state batteries37. These pioneering research works ushered a new direction for developing soft solid electrolytes and circumventing the surface contact issue in solid-state batteries with inorganic solid electrolytes./p>10−3 S cm−1) for operating SPE-based SSLMBs at elevated temperatures (≥80 °C)51,52. In the last decade, the development of molecules with delocalized negative charges has further progressed53,54. For example, Ma et al.54 proposed a delocalized polyanion, i.e., poly[(4-styrenesulfonyl)(trifluoromethyl(S-trifluoromethylsulfonylimino) sulfonyl)imide] (PSsTFSI−), that demonstrates improved lithium-ion conductivity of SPEs for unipolar conduction (i.e., only positive charges are mobile) due only to lithium cation (e.g., at 80 °C, ca. 10−4 S cm−1 for LiPSsTFSI-based electrolyte and ca. 10−5 S cm−1 for lithium poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide] (LiPSTFSI)-based electrolyte54). The polyanion PSsTFSI− could be obtained through the replacement of an oxygen atom in a TFSI-like moiety (i.e., CF3SO2N(−)SO2—) with strong electron-withdrawing trifluoromethanesulfonylimino ( = NSO2CF3) group; thus, the negative charges are further delocalized via five oxygens and two nitrogen atoms. These research works demonstrate an effective strategy for improving the ionic conductivity in coupled SPEs by weakening the interaction between salt anion and lithium ions./p>50 vol%) SPEs or when particular morphologies (e.g., nanowire) of inorganic phases are used56./p>50 wt% of salt in SPEs)17, promoting the metal ions to diffuse through this second conduction path. This demonstrates the ability of PIS-type SPEs to decouple metal-ion motion from polymer dynamics77. Several criteria, such as polymer Tg, salt type, polymer/salt solubility, electrochemical stability, and ionic conductivity78, were also discussed in C. Austen Angell’s early works to understand the physicochemical properties of PIS electrolytes. Among these, the concept of the ionicity of lithium salt is of utter importance. Specifically, ionicity is a measure of the degree of ion dissociation, commonly referring to the effective fraction of ionic species being able to participate in ionic conduction18. Figure 5c displays the Walden-Angell plot for the dependence of equivalent conductivities on the viscosities of electrolytes. With 1.0 M potassium chloride/H2O solution as a reference electrolyte, the regime above the diagonal line refers to the electrolyte materials with super-ionic characters. For PIS-type SPE systems, the lithium salt should possess sufficient ionicity to ensure the high conductivity, i.e., be located in the super-ionic regime in Fig. 5c./p> 4089) via the loose structures (i.e., rigid polymer chains with low packing density), despite their low segmental relaxation rate; segmental motion is necessary for the less-fragile polymers with dense structures (i.e., compact packing of flexible polymer chains), including PEO and other polyethers./p>0.9). This is also a fact in most PIS-type electrolytes since only a small portion of metal ions can be decoupled from the polymer, whereas the rest are still bound to the polymer chains. In the case of the PolyIL-IS systems, the weak coupling between the metal ion and the polymer exists through the anion-bridging co-coordination. The highly coupled metal ion-anion motion also limits the metal-ion transference number to ca. 0.595. In this case, improving the ionicity of the salt could maximize the decoupling motion in PolyIL-IS, although not yet experimentally proved./p>1000 cycles) and stable cycling of these SPE-SSLMBs have been achieved by the research group at Hydro-Québec100. Yet, the main obstacle to large-scale implementation of batteries with vanadium-based positive electrodes lies, at the cell level, in the dissolution of vanadium species during continuous cycling, and at the raw material level, in the uneven geographical distribution of vanadium resources worldwide101./p>6 × 108 km with a decent safety record (only two cases with unexplained runaway reactions). These industrially-relevant examples stimulated industrial and academic laboratories to restart the research activities in lithium metal rechargeable batteries after the initial abandonment of this technology as a consequence of the various fire accidents that occurred in AA-size Li°||molybdenum disulfide cells produced by Moly Energy in the late 1980s24./p>350 °C (LiCF3SO3)]115. A further homologation of the oxygen atom results in the formation of a super lithium sulfonimide salt (Li[CF3SO(NSO2CF3)2], LisTFSI) with a low melting transition approaching the ionic liquid domain (i.e., Tm ≤ 100 °C for typical ionic liquids88, and Tm = 118 °C for LisTFSI116). In this regard, we speculate that the concept of negative charge delocalization could be extended further to accessing liquid lithium salts. From another perspective, one may also replace typical neutral polyether/polyesters with charged polysalts (e.g., polycations, polyanions, or poly(zwitterions)), to regulate the ion-ion interactions and thereby achieving decoupled SPE systems117. For instance, the utilization of imidazole-type poly(zwitterions) could provide ordered subdomains with superionic nature, which allows rapid transport of ionic species even at temperatures close to their Tg values118./p>

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