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.

References

M. Thackeray et al., J. Mater. Chem.
17, 3112–3125 (2007).

N. Yabuuchi et al., PNAS
112, 7650–7655 (2014).

J. Lee et al., Nature
556, 185–190 (2018).

Seo et al., Nature Chem.
8, 692–697 (2016).

H. Ji et al., Nat. Commun.
10, 592 (2019).

R. House et al., Nature
577, 502–508 (2020).

Z. Lun et al., Adv. Energy. Mater. 1802959 (2019).

J. Lee et al., ACS Appl. Energy Mater.
3, 7931–7943 (2020).