A Surface Se‐Substituted LiCo[O_2−δSe_δ] Cathode with Ultrastable High‐Voltage Cycling in Pouch Full‐Cells

Abstract

Stabilizing high-voltage LCO cycling is a hot topic in both academic and industrial research. [3,4] However, the exact mechanism that caused the quick fading of high-voltage LCO has not yet reached consensus. [5,6] The band energy diagram in Figure S1 in the Supporting Information shows that cycling LCO to high voltage must entail a hybrid O anion (O 2− →O α− , α < 2) and Co cation-redox (HACR). [7,8] It is tempting to "exploit" HACR in LCO for much higher capacity, e.g., if LCO is charged to above 4.6 V, more than 220 mAh g −1 can be obtained; however, because of the reduced ionic radius and electrostatic force, the oxidized O α− would become much mobile [9] and more likely to escape from the particle, resulting in oxygen loss (OL). Continuous OL can be a killer problem to high-voltage cycling. [10] First, OL causes irreversible phase transformations (CoO 2 →Co 3 O 4) [11] (Figure S2, Supporting As the pioneer cathode for rechargeable Li-ion battery, [1] LiCoO 2 (LCO) is still dominating today's battery markets in consumer electronic devices, due to its high volumetric energy density and stable cycling. However, as LCO is only cycled within 4.35 V and 165 mAh g −1 at the present to meet the industrial-level cycling life, [2] there is still a large space to increase its utilizable capacity by charging LCO to higher voltages before it reaches

Tailoring Co3d and O2p Band Centers to Inhibit Oxygen Escape for Stable 4.6 V LiCoO2 Cathodes

“…Lu et al reported a class of ternary lithium, aluminum and fluorine‐modified LiCoO 2 with a stable and conductive layer [6] . Li et al improved the cycling stability through Se substitution(Se‐LCO), and Se‐LCO retained 80 % of its capacity after 450 cycles at 100 mA g −1 in 4.57 V pouch full cells [7] . Recently, Huang et al reported a 4.6 V Mg‐pillared LiCoO 2 with a capacity retention of 84 % at 1.0C over 100 cycles [8] .…”

“…It has been reported that oxygen redox (O 2− ↔O 2 n − ) begins to contribute to capacity at a higher voltage due to O 2p orbital hybridization with the Co 3d orbital in the Co 3+/4+ :t 2g and O 2p resonant bands at lower electronic energies [10] . The electrons are extracted from both Co 3+ and O 2− , which leads to O 2− oxidation and oxygen loss from the cathode materials due to the decrease in the ion radius and electrostatic force, especially at a high potential [7, 11, 12] . Moreover, the formed oxygen, including O 2 gas and O − radicals is highly oxidizing, which rapidly decomposes the carbonate electrolyte and produces a thick cathode electrolyte interface (CEI) that affects the cycling performance of the batteries.…”

Stabilizing electrode–electrolyte interfaces to realize high-voltage Li||LiCoO2 batteries by a sulfonamide-based electrolyte

“…9,10 On the one hand, the undesirable electrolyte decomposition 11 including solvent oxidation and hydrogen abstraction 12 from solvent molecules leads to the formation of high-impedance CEIs generally in the form of oxyfluorides/oligomers. 13 On the other hand, the parasitic electrolyte-cathode reaction promotes further degradation, including the formation of ionically resistive spinel 14 transformed from the layered structure, Co dissolution in the electrolyte 15,16 and O loss 17 from oxygen evolution. Therefore, constructing stable CEIs to mitigate these degradations is required.…”

“…Prevailing strategies for protecting LCO are bulk doping 7,8,18 and surface modification. 5,9,15,17,[19][20][21][22][23] An alternative strategy is to develop highly compatible electrolytes to form stable CEIs. Although fluorinated electrolytes, 24,25 multifunctional polymer electrolyte, 26,27 and additives 28,29 have demonstrated promising cyclability improvement, such explorations have been focused mainly on charging voltages up to 4.5 V Li 24,25,30 (Table S1, ESI †).…”

Sublimated Se‐Induced Formation of Dual‐Conductive Surface Layers for High‐Performance Ni‐Rich Layered Cathodes

“…As present in Figure.3e, two peaks located at 60.58 eV and 61.78 eV are clearly observed, which can be related to the 3d 5/2 and 3d 3/2 signals of Se 6+ in Li 2 SeO 4 , indicating the formation of a uniform Li 2 SeO 4 coating on the external surfaces of Ni‐rich secondary particles [31] . Next, as the calcination temperature is increased and calcination time is prolonged, gas‐phase Se tends to penetrate the secondary particles, thereby the precoated Se will initially substitute some lattice oxygen to anchored Se as SeO 2 [32] . The existence of SeO 2 can be proved by the 3d 5/2 (59.06 eV) and 3d 3/2 (60.05 eV) signals of Se 4+ [33] .…”