On the Composition of LiNi_0.5Mn_1.5O₄ Cathode Active Materials

Abstract

A characteristic advantage over state-of-the-art Ni-rich cathode active materials such as LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM-622) or LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM-811) is that LNMO is cobalt free and the redox capacity of nickel is used in its entirety. The latter could become more important considering rising nickel prices. Based on realistic full cell capacities of 120 mAh g −1 for LNMO and 170 mAh g −1 for the NCM materials, 43.8 Ah per mol Ni is obtained for LNMO, whereas NCM-622 and NCM-811 only yield 27.5 and 20.7 Ah mol −1 , respectively. The specific energy density of LNMO vs. graphite full cells is likewise highly competitive. For LNMO, up to 464 Wh kg −1 are expected, which exceeds the energy densities of corresponding NCM-622 and NCM-811 cells, yielding 416 and 448 Wh kg −1 , respectively. [2] Despite these favorable features, commercialization of LNMO cathode materials never succeeded. Up to date, it was not possible to build competitive cells with graphite anodes, as the degradation of these full cells has not been brought under control. [3,4] The problem, however, is not the stability of LNMO itself, but the dissolution and disproportionation of Mn(III) from LNMO, which leads to continuous decomposition reactions on the anode side and ongoing loss of cyclable lithium. [3,5,6] LNMO is nevertheless a very stable cathode material, which does not tend to decompose or release oxygen during cycling. [7] There is scientific consensus that LNMO crystallizes as an ordered or disordered spinel phase in the space groups P4 3 32 or Fd3m, respectively, or as intermediate, partial ordered phases. The term "order" refers to the distribution of transition metal cations on the octahedral sites of the spinel structure. In the ordered structure, Mn and Ni predominantly occupy the Wyckoff positions 12d and 4a, while in the disordered structure, they are statistically distributed over the 16c octahedral site. [8,9] Whether an LNMO material is predominantly ordered or disordered is commonly associated with the calcination temperature. Temperatures below 730 °C are reported to favor the formation of the ordered phase, whereas at higher temperatures disordered crystal structures are reported to occur. [10] LNMO phases with the composition LiNi 0.5 Mn 1.5 O 4 , where Mn is Mn(IV) exclusively, are reported to be obtained by calcining LNMO materials at 700 °C. [11] Increasing calcination temperatures however, lead to a partial reduction of Mn(IV) to Mn(III), which becomes obvious from an increasing Mn 3+ /Mn 4+ LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode active materials for lithium-ion batteries have been investigated for over 20 years. Despite all this effort, it has not been possible to transfer their favorable properties into applicable, stable battery cells. To make further progress, the research perspective on these spinel type materials needs to be updated and a number of persisting misconceptions on LNMO have to be overcome. Therefore, the current knowledge on LNMO is summarized and controversial points are addressed by detaile...

Cycling stability of lithium-ion batteries based on Fe-Ti doped LiNi0.5Mn1.5O4 cathodes, graphite anodes and the cathode additive Li3PO4

“…Simultaneously, however, superior full cell cycling stability for the LNMFTO material containing only 4% Mn III was found [29]. It has also been shown that disordered LNMO materials with significantly reduced Mn III content can be obtained and it was suggested that such materials should be best suited for the use in LNMO vs. graphite full cells [8].…”

“…Despite many efforts, there was still a lot of contradictions and confusion about LNMO, for example concerning the Mn III content, cation order, secondary phases and allegedly existing oxygen defects. Several aspects have only recently been clarified on the basis of systematic experimental work and an exact chemical equation for the formation of LNMO [8]. The main reason for the lack of commercial success of LNMO materials, however, still is considered to be the strong degradation in LNMO/graphite cells.…”

“…Unfortunately, the vast majority of disordered LNMO materials found in literature are quite Mn III rich. Typical LNMO phase compositions of such disordered LNMO materials are ranging from LiNi0.46Mn1.54O4 to LiNi0.43Mn1.57O4 and thus contain 5% to 9% Mn III [8]. Mn III , however, is generally assumed to play a central role in full cell degradation.…”

“… Despite a theoretical specific capacity in the case of LNMO of 147 mAh•g -1 , the practical specific capacity is barely more than 120 mAh•g -1 as a result of secondary phases of LNMO materials [8] and Li consuming solid electrolyte interface (SEI) formation (see below).  The loading of the electrolyte material is often limited due to the damage of LNMO electrodes during cycling, e.g.…”

Unraveling the effects of inter-site Hubbard interactions in spinel Li-ion cathode materials

“…16,17 It is known experimentally that LiMn 1.5 Ni 0.5 O 4 also crystallizes in a spinel phase but in two possible space groups Fd% 3m or P4 3 32 (or P4 1 32, which is an enantiomorph of P4 3 32) depending on the synthesis conditions, and that Ni 2+ is redox active while Mn 4+ is inactive. [17][18][19] This material adopts a ferrimagnetic (FiM) ordering below the Curie temperature (B130 K) in which the Mn 4+ spins align antiparallel to the Ni 2+ spins. 15,20,21 Nonetheless, the commercialization of this material has been hampered by severe capacity fade, particularly at elevated temperatures, in cells employing a graphite anode.…”