Prospects and limitations of single-crystal cathode materials to overcome cross-talk phenomena in high-voltage lithium ion cells

The specific energy of lithium ion batteries can be further enhanced by increasing the cell voltage (>4.3 V). However, conventional cathode active materials (CAMs) e.g. LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) with typical poly-crystal (PC)-based secondary particles suffer from rollover failure at 4.5 V, which is shown to be the result of an electrode cross-talk, i.e., dissolution of transition metals (TMs) from the cathode and deposition at the graphite-based anode. Interestingly, the TM deposits at the anode are locally accumulated and dendritic Li deposits are analytically indicated on exactly these spots. Severe formation of Li dendrites is concluded to be the onset of sudden and abrupt capacity fade as it is accompanied by severe consumption of active Li. In contrast, NCM523 CAMs based on single-crystals (SCs), which are single-standing primary particles, demonstrate an improved cycle life in SC-NCM523‖graphite cells. Less rollover fading, cross-talk and Li dendrites at the anode are observed and attributed to the morphology of the SC-based cathode. It is concluded that the lower specific surface area diminishes electrolyte contact, thus the reaction area for transition metal dissolution and finally improves the high voltage performance.

Introduction

Enhancing the operation potential of positive electrodes (cathodes) is a common strategy to increase the cell voltage, and thus specific energy and power of batteries.^1–3 State-of-the-art cathode active materials (CAMs) for lithium ion batteries (LIBs) are layered oxides with the typical composition of LiNi_xCo_yMn_zO₂ (NCMxyz; x + y + z = 1), e.g., LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523).^4–8

It is of particular interest to further enhance the charge potential of NCM cathodes above the common limit of 4.3 V vs. Li|Li⁺ and thus the overall cell operation voltage. This comes with a further increase in specific capacity, which additionally boosts the specific energy.^9,10 However, this approach is accompanied by structural instabilities of the NCM materials, followed by parasitic decomposition reactions and severe capacity fading.^9,11,12 Above the potential of 4.3 V vs. Li|Li⁺, a thermodynamically driven decomposition converts the structure of layered oxides to spinel phases and finally to a thermodynamically more stable, but inactive, rock-salt phase.^13,14 These phase changes are intertwined with detrimental oxygen release and transition metal dissolution (i.e., Co, Ni and Mn) into the electrolyte.^15–18 Though, only in marginal amounts relative to NCM,^11,19 the dissolved transition metals can deposit at graphite-based negative electrodes (anodes) and deteriorate the solid electrolyte interphase (SEI)^20–23 in the course of the well-known electrode or electro-(chemical) cross-talk.^15,24–32 As a consequence, this triggers losses of active Li via formation of high surface area lithium (HSAL), e.g. in the morphological form of dendrites and further decreases the cycle life of LIBs.^15,24 In a previous publication it is shown that this can lead to rollover failure in NCM523‖graphite cells.³³

In addition, also the macroscopic instability of NCM can additionally promote detrimental cross-talk. In general, NCM materials are based on micron-sized secondary particles, which are aggregated by numerous nano-sized primary particles.^34–36 It is known in the literature that secondary particles can crack, in particular promoted by the strain at high voltage operation (“electrochemical shock”).^34,37 The increased surface area accompanied by increased exposure to the electrolyte can further promote transition metal (TM) dissolution.^14,38,39 These secondary particles, being prone to inter-granular cracking, are denoted as ‘poly-crystal (PC)’-based active materials.^14,34

A smart approach to circumvent inter-granular cracking and to minimize the overall surface area of CAMs is the use of non-agglomerated, thus single, primary particles with an enhanced particle size (≈5 μm).^40,41 These are denoted in the literature as ‘single-crystal (SC)’ with reported superior performance for various SC-based CAMs, including NCM523, NCM622 and NCM811.^42–48 Moreover, in line with calculations from fracture mechanics, larger primary particles are supposed to be mechanically more stable in regard to intra-granular cracking compared to smaller particles.^14,49,50 This overall mechanical stability is beneficial also in terms of electrode processing as it renders them less prone to cracking, e.g. during pressing, and can realize higher electrode densities, and thus higher areal capacities.^14,51

In this work, the behavior of a SC-NCM523-based CAM is thoroughly investigated in a NCM523‖graphite full-cell under high voltage conditions (upper cell voltage: 4.4 V to 4.7 V) in contrast to conventional PC-NCM523. Significant performance differences due to appropriate morphological alteration are highlighted and discussed with the support of electrochemical and analytical methods.

Full article: Journal of Materials Chemistry