Kinetic Rejuvenation of Li-Rich Layered and Disordered-Rocksalt Li-Ion Battery Cathodes upon Oxygen Redox
Authors: Lee Jinhyuk , Li Ju 
Published: July 2021

Li-rich cathode materials (x>1 in LixTM2-xO2, TM=transition metal) with a layered structure or a disordered-rocksalt structure have received tremendous attention as advanced battery cathodes [1,2,3]. These materials can deliver substantially higher gravimetric capacity (>250 mAh/g) and energy density (>800 Wh/kg) compared to traditional materials (e.g., LiCoO2; ~170 mAh/g, 660 Wh/kg). Moreover, they improve the Li-ion batteries' sustainability by removing or greatly reducing scarce and environmentally unfriendly elements in their structure. In these materials, the theoretical TM-redox capacity is typically limited to ~140 mAh/g because of the limited amount of redox-active TMs in their crystal structure [1,2,3]. As a result, to achieve a very high capacity (> 250 mAh/g), O-redox must additionally take place in the materials, which occurs in local Li-rich environments (e.g., OLi4TM2, OLi5TM1) around oxygen where there are non-bonding O 2p orbitals [4]. In Li-rich materials containing multiple TMs, the local electroneutrality demands the TM-sites in the OLi4TM2 or OLi5TM1-unit to be occupied by high-valent TM-cations (e.g., Ti4+, Mn4+) instead of low-valent ones (e.g., Ni2+), which often results in medium-range-order (MRO) of cations in the Li-rich layered cathodes or short-range-order (SRO) in the Li-rich disordered-rocksalt cathodes [1,5]. For instance, this tendency leads to the rise of the Li2MnO3-like MRO in the layered Li- and Mn-rich cathodes, and because the non-bonding O 2p orbitals are primarily in the Li2MnO3-like domains, O-redox also takes place within the domains [1,4].

Unfortunately, the radical oxygen ion generated upon oxygen oxidation is highly mobile, and as the oxygen is covalently bonded with TMs, this increased O-mobility often accompanies TM-migration, triggering reversible or irreversible structural changes in the Li-rich materials [1]. This structural changes result in oxygen loss or unwanted phase transformation, which leads to voltage- and capacity-decay. Therefore, minimizing the structural rearrangement while using combined TM- and O-redox has been widely considered a holy grail to developing Li-rich cathodes with high capacity and stability. For instance, Bruce et al. showed that by changing the superstructure from the honeycomb to ribbon structure, one could suppress the Mn-migration upon O-redox in the layered Na-(Li, Mn)-O cathode to reduce voltage hysteresis [6]. Moreover, Li-rich disordered-rocksalt cathodes also experience structural damages upon O-oxidation; hence, its mitigation by various methods (e.g., fluorination) has been at the center of its research [3,7].

In this presentation, we demonstrate that, in contrast to the prevailing opinion, the O-redox-assisted structural change can be highly beneficial to Li-rich cathodes' performance by substantially reducing the internal Li-transport resistance [8]. We synthesized highly-Li-rich Co-free layered cathodes, which possess the Li2MnO3-type medium-range-order (MRO). These materials' initial capacities are low; yet, they undergo a 'rejuvenation' (activation) upon extended cycling with substantially reduced hysteresis and increased capacity, which correlates with the MRO disruption and associated volume expansion initiated by O-redox-facilitating TM-migration. Moreover, we show that virtually the same process occurs for a disordered-rocksalt-type Li-rich cathode (Li1.2Ni1/3Ti1/3Mo2/15O2), suggesting the universality of this process.

Furthermore, we use this knowledge to inform a molten-salt treatment to pre-disturb MRO, expand crystal volume before cycling, and endow a surface gradient composition for our Li-rich Co-free layered cathodes, such that the treated materials can achieve high capacity (>230 mAh/g) from the very first cycle with excellent rate capability (154 mAh/g at 2 A/g) and outstanding capacity/voltage-retention (~4 % capacity-loss; ~140 mV voltage-loss after 200 cycles at 100 mA/g; >210 mAh/g). From these results, we explain the mechanism and universality of the rejuvenation process in various charge-ordered oxides and propose guidelines for designing advanced Li-rich cathode materials with combined transition metal- and oxygen-redox activities.

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