# Li2O-Based Cathode Additives Enabling Prelithiation of Si Anodes
## 1 Introduction  
Silicon (Si) is a promising next-generation anode material for lithium (Li)-ion batteries (LiBs) owing to its high theoretical specific capacity (3579 mAh/g upon formation of Li\({}_{15}\)Si\({}_{4}\)) and relatively low operating voltage (\(\sim\)0.4 V vs. Li/Li\({}^{+}\)) [1, 2, 3]. Its alloying chemistry, however, results in large volume change (\(\sim\)280\(\%\)) during operation, which leads to pulverization and loss of active material. To mitigate the mechanical failure of electrodes, Si is typically utilized as nanometer-sized particles [4, 5]. Such small particles, however, create extremely high surface area and, as a result, Si-based electrodes suffer from significant Li consumption during the formation of the solid electrolyte interphase (SEI) in the initial cycles. One possible solution to this problem is supplying excess Li in the system, i.e., prelithiation, to compensate for the initial loss [6, 7, 8]. For example, additional Li can be chemically incorporated into Si anodes by immersing the electrode in reactive agent solutions such as Li-reduced polyaromatics [9, 10] or bringing it into direct contact with Li metal [11, 12]. Alternatively, Si anodes can be electrochemically prelithiated by cycling the electrode in an electrochemical cell using Li metal counter electrode [13, 14]. These methods supply Li to the anode, and this additional Li can compensate for the loss during the initial charge by providing sufficient capacity during the reverse (discharge) cycle. Alternatively, additional Li can be introduced to the cathode to provide the capacity required for the SEI formation. For example, the cathode-active material can be over-lithiated via chemical orelectrochemical processes [15; 16], or additives capable of releasing Li during charge can be incorporated [17; 18; 19; 20; 21; 22; 23; 24; 25]. Among these methods, utilizing cathode additives is a convenient approach, as it does not require an additional step in the battery-manufacturing process (the additives can be directly incorporated into the cathode slurry) nor a complicated synthetic process to alter the cathode-active materials. Different additives, such as lithium nitride (Li\({}_{3}\)N) [17; 18], anti-fluorite Li\({}_{5}\)FeO\({}_{4}\) (LFO) [19; 20], lithium sulfide (Li\({}_{2}\)S), [21; 22], and lithium oxide (Li\({}_{2}\)O) [23; 24; 25] have been explored. While all these additives have shown promise, the chemical instability of Li\({}_{3}\)N, LFO, and Li\({}_{2}\)S brings challenges. Li\({}_{2}\)O is a safe, cost-effective candidate that can release Li\({}^{+}\) during the first charge half-cycle and remain inactive in the following cycles, due to the large hysteresis of the reverse reaction [23; 24]. However, since Li\({}_{2}\)O is electrically insulating, activating this material and obtaining the additional capacity provided by this Li\({}^{+}\) source are nontrivial. Previous work by [PERSON] et al. showed nanocomposites of Li\({}_{2}\)O and Co created by reacting Co\({}_{3}\)O\({}_{4}\) with molten Li metal can deliver additional capacities at voltages below 4.1 V vs. Li/Li\({}^{+}\) upon charging [23]. In addition, [PERSON] et al. utilized ball-milled, micro-sized Li\({}_{2}\)O powder and demonstrated that the additive performance depends on the charging rate, electrolyte, and the anode [24]. Recently, [PERSON] et al. reported anode-free Li-metal batteries where the cathode (LiNi\({}_{0.8}\)Co\({}_{0.1}\)Mn\({}_{0.1}\)O\({}_{2}\)) was coated with Li\({}_{2}\)O, and fluorinated ether electrolyte additive was used to create a lithium fluoride (LiF)-rich cathode electrolyte interface (CEI), suppressing reactions between O\({}_{2}\)' species and the electrolyte [25]. Thus, there is a clear trend of growing interest in utilizing Li\({}_{2}\)O as a cathode additive for prelithiation. In this work, we evaluated how to effectively utilize the cathode Li\({}_{2}\)O additive, specifically for prelithiating Si anodes.