Uma reflexão sobre eletrólitos poliméricos para sólidos

Nature Communications volume 14, número do artigo: 4884 (2023) Citar este artigo

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Antes da estreia das baterias de íon-lítio (LIBs) no mercado de commodities, as baterias metálicas de lítio de estado sólido (SSLMBs) eram consideradas sistemas promissores de armazenamento de energia eletroquímica de alta energia antes de serem quase abandonadas no final da década de 1980 devido a questões de segurança. No entanto, após três décadas de desenvolvimento, as tecnologias LIB estão agora a aproximar-se do seu conteúdo energético e dos limites de segurança impostos pela química da cadeira de balanço. Estes aspectos estão a estimular o renascimento das actividades de investigação em tecnologias SSLMB, tanto a nível académico como industrial. Neste artigo de perspectiva, apresentamos uma reflexão pessoal sobre eletrólitos poliméricos sólidos (SPEs), abrangendo desde o desenvolvimento inicial até sua implementação em SSLMBs, destacando marcos importantes. Em particular, discutimos as características das SPEs levando em consideração o conceito de SPEs acopladas e desacopladas proposto por C. Austen Angell no início da década de 1990. Possíveis soluções para melhorar as propriedades físico-químicas e eletroquímicas dos SPEs também são examinadas. Com este artigo, também pretendemos destacar os blocos que faltam na construção de SSLMBs ideais e estimular a pesquisa de materiais eletrolíticos inovadores para futuras baterias recarregáveis de alta energia.

Em seu romance de 1870 “Vinte Mil Léguas Submarinas” Júlio Verne descreve o submarino Nautilus como alimentado por um sistema de bateria avançado, e o Capitão Nemo menciona que “…as células com sódio devem ser consideradas como as mais energéticas, e que sua força eletromotriz é o dobro das células de zinco.”1 O conceito de construção de baterias de alta energia proposto por Júlio Verne estava sem dúvida à frente do seu tempo no final do século XIX, mas em linha com o fascínio então pelas maravilhas geradas pelo uso da eletricidade. “Tudo com eletricidade” era um sonho da humanidade que vivia no início dos anos 1900, mas se tornaria realidade no final do século 20 com a invenção das baterias de íons de lítio (LIBs) baseadas no conceito de cadeira de balanço (ou seja, baterias construídas com dois eletrodos baseados em intercalação com diferentes potenciais para armazenamento/entrega de energia eletroquímica)2,3. Atualmente, a produção global de LIBs atingiu uma grande escala de >500 gigawatts-hora (GWh), atuando como fontes de energia para mais de 6 milhões de veículos elétricos (EVs)4. O sucesso dos LIBs atesta a hipótese inicial de “todos com electricidade” e abre novos caminhos para um desenvolvimento mais sustentável de actividades antropogénicas consumidoras de energia.

A capacidade de produção de LIBs aumentou dez vezes ao longo da última década5, e espera-se que esta procura continue a crescer rapidamente durante os próximos 10 a 30 anos, impulsionada principalmente pelo setor de VE em rápido crescimento4. A necessidade de baterias recarregáveis de alto desempenho (por exemplo, densidade de energia, segurança, custo, etc.) também é premente, especialmente considerando os requisitos rigorosos trazidos pelas aplicações práticas contemporâneas (por exemplo, VEs para estrada e voo, drones, robótica avançada, etc.). .), incluindo segurança inerente e energia específica (>500 Wh kg−1) e densidade de energia (>1000 Wh L−1)6. Infelizmente, os eletrólitos líquidos não aquosos usados nos LIBs atuais são instáveis e altamente inflamáveis devido à presença de solventes de carbonato orgânico (por exemplo, carbonato de dimetila, carbonato de etileno, etc.); além disso, os eletrodos negativos de grafite com capacidade específica relativamente baixa de 372 mAh g-1 também são fatores limitantes para melhorar ainda mais a energia específica dos LIBs do estado da arte6. A este respeito, baterias de metal de lítio de estado sólido (SSLMBs) acoplam materiais de eletrodo de alta energia (por exemplo, metal de lítio (Li°), ligas de lítio, LiNi1−x−yCoxMnyO2 rico em níquel (1−x − y > 0,8), enxofre, etc.) com eletrólitos sólidos são considerados uma abordagem viável para contornar o obstáculo específico da densidade de energia da atual tecnologia LIB7,8,9.

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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