Surface Mn Oxidation State Controlled Spinel LiMn₂O₄ as a Cathode Material for High‐Energy Li‐Ion Batteries

Abstract

electrochemical cycling at elevated temperatures owing to Mn 2+ dissolution into the electrolyte (2Mn 3+ solid → Mn 4+ solid + Mn 2+ solution ), [ 2 ] electrolyte decomposition at high voltages, [ 3 ] and local Jahn-Teller distortions of the Mn 3+ ions during high rate discharge. [ 4,5 ] Various methods have been investigated to overcome these problems, such as doping with foreign ions (Li 1+ x Mn 2− x − y M y O 4 , M = Li, [ 6 ] Co, [ 7 ] Mg, [ 8 ] Al, [ 9 ] etc.), surface coatings, particle size control, cathode material blending, [ 10 ] and the utilization of HF free electrolytes, [ 11 ] electrolyte additives, [ 12 ] and nongraphitic anodes.Since the degradation in cell performance originates from the electrode materials, direct cathode treatments offer a more direct and potentially satisfactory solution. In this regard, cation substitution is the approach that has attracted most attention, and has indeed been adopted in industry to enhance the cycleability of LiMn 2 O 4 batteries. However, doping these materials brings about large reductions in capacity, [ 13 ] such that in practice, the capacities of commercial spinel materials for Li-ion batteries are only ≈70% of that available in theory, canceling this particular merit of LiMn 2 O 4 . To avoid direct contact between the active materials and the electrolyte, the cathode can be coated with a thin, highly stable layer, thereby enhancing the cycleability of the device without signifi cantly degrading its capacity. Various metal oxide coatings such as Al 2 O 3 , [ 14 ] AlPO 4 , [ 15 ] SiO 2 , [ 16 ] SnO 2 , [ 17 ] MgO, [ 18 ] LiCoO 2 , [ 19 ] etc., have been studied in this context. However, the interface between the coating and the host is not seamless, so that any tiny gaps between the two can be formed, resulting in electrolyte penetration. The electrochemical performance of these coatings is therefore insuffi cient for Li-ion battery applications.The most important criteria for a LiMn 2 O 4 coating material to satisfy are: (1) to cover a large portion of the host surface effectively, (2) to remain stable during cycling, and (3) to allow Li-ion diffusion. Cation-substituted spinel oxides such as Li 1+ x M n 2− x − y M y O 4 , fulfi ll these requirements; indeed, (1) they have the same spinel structure as stoichiometric LiMn 2 O 4 and form epitaxial layers (rather than segregated particles) on the bulk surface, thereby providing good coating coverage, (2) cation-substituted spinels with an average Mn oxidation state greater than 3.6 cycle well at elevated temperatures, and (3) they are electrochemically active therefore potentially good ionic conductors. [ 20 ] For example, Li 1.15 Co 0.32 Mn 1.53 O 4 , which was used in this paper Spinel lithium manganese oxide (LiMn 2 O 4 ) has attracted much attention as a promising cathode material for large-scale lithium ion batteries. However, its continuous capacity fading at elevated temperature is an obstacle to extended cycling in large-scale applications. Here, surface Mn oxidation state controlled LiMn 2 O 4 is ...

Environmentally stable interface of layered oxide cathodes for sodium-ion batteries

“…4. According to previous results, the Ti-2p 3/2 peaks at 456.6 and 458.3 eV simulated by a fitting process are attributed to Ti 3+ and Ti 4+ , respectively; 38 a Mn-2p 3/2 peak is observed at 641.5 eV, which is representative of Mn 3+ ; 39 the Ni-2p 3/2 peak located at 855.6 eV confirmed the presence of Ni 2+  40 . Given that the XPS and ELNES techniques are relatively limited to the particle surface, these results mainly reflect the surface characteristics of the NMTN samples, and the core region, which possibly contains other valence states, such as tetravalent manganese, cannot be detected at present.

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Prospect and Reality of Ni‐Rich Cathode for Commercialization

“…[35][36][37][38][39] The incorporated coating precursor could react with the residual lithium compounds during annealing process, thereby diminishing these compounds. This innovative approach makes the synthetic process for the nickel-rich cathodes highly efficient because of the absence of the washing and additional calcination process, decreasing the capital cost and process time (Figure 3).…”

Enhanced High‐Temperature Electrochemical Performance of Layered Nickel‐Rich Cathodes for Lithium‐Ion Batteries after LiF Surface Modification

“…[24][25][26][27] To overcome these limitations, surface modification is one of most commonly employed routes, which can suppress side reactions at the electrode/electrolyte interface, thereby improving cycle stability. [28][29][30] Currently, many researches have discussed the coating of oxides, [31] phosphates, [32] carbon, [33] and fluoride. [34] In particular, fluoride is considered to effectively protect the electrode material from HF etching, and lithium fluoride has higher Li ion conductivity in comparison with other fluorides.…”