Background introduction
The nickel rich material linixcoymnzo2 (x + y + Z = 1, X & gt; 0.6) has the advantages of high operating voltage, large capacity, low cost, non-toxic and so on. It is the best choice for the positive electrode of lithium-ion battery (LIBS). However, the structure of nickel rich materials will change under high working voltage. At the same time, the side reaction between electrolyte and high activity Ni4 + will cause the electrolyte to oxidize and decompose, which hinders its commercial application. In order to improve the structural stability and electrochemical performance of nickel rich anode, the stability of electrolyte should also be considered.
The electrolyte of commercial lithium-ion batteries usually consists of 1 m LiPF6 dissolved in a mixture of cyclic and linear carbonates. LiPF6 is widely used because of its high ionic conductivity, good oxidation resistance and good compatibility with aluminum collector fluid. However, LiPF6 has low thermal stability and can react with trace water to form acidic compounds such as PF5 and HF. These acidic compounds will reduce the stability of SEI, and HF will lead to the dissolution of metal ions in the positive material, which will greatly damage the battery performance.
It is reported that compounds containing trimethylsilyl (TMS) can effectively capture f-ions from HF. In addition, N atom can form coordination complex with strong Lewis acid PF5. Therefore, the combination of TMS and N-containing groups is considered to be a very effective way to improve the stability of LiPF6 based electrolyte in LIBS. However, although aminosilane (n-Si) can remove HF and H2O, its mechanism remains to be explored.
Introduction of results
Recently, sang kyu Kwak and Nam soon Choi from the National Institute of science and technology of Ulsan, South Korea, introduced 3 - (trimethylsilyl) - 2-Oxazolidinone (TMS The influence of tms-on on was systematically analyzed from the following aspects: 1) the stability of LiPF6 based electrolyte; 2) the stability of electrode / solution interface; 3) the microstructure change of lini0.7co0.15mn0.15o2 (NCM) as a nickel rich cathode material during the cycle. Calculation analysis and nuclear magnetic resonance (NMR) test showed that tms-on promoted the dissociation of LiPF6 by coordination with Li +, reduced the hydrolysis of LiPF6 by stabilizing PF5, and eliminated HF. In addition, tms-on produced 2-Oxazolidinone (on) after HF removal, which changed the interface structure between NCM positive electrode and graphite negative electrode appropriately. In addition, by means of XPS and HR-TEM, it is shown that SEI derived from on can alleviate the decomposition of electrolyte near the NCM positive interface and the irreversible phase transition of NCM positive electrode. Relevant research results were published in adv. energy mater. Under the title of cyclic aminosilane based additional ensuring stable electron – electronic interfaces in Li ion batteries.
Graphic analysis
LiPF6 is a commonly used lithium salt in LIBS electrolyte. It exists in many forms, including the ion pair LiPF6 which is not completely dissociated by solvent molecules, and the solvated Li + and PF6 -. The ion pair of LiPF6 is prone to Autocatalytic Decomposition, forming LIF and PF5, and the higher the temperature, the easier the decomposition. PF5 is a strong Lewis acid, which can cause the decomposition of cyclic carbonate. In addition, PF5 reacts with water to form pof3, HF and phosphoric acid compounds with high activity (hpo2f2, h2po3f and H3PO4). In addition, hpo2f2 and HF will be generated when the PF6 - formed by LiPF6 decomposition reacts with water. HF will not only lead to the dissolution of transition metal ions in the positive material, but also cause serious damage to the negative SEI. Therefore, inhibiting the formation of the active substances (HF and PF5) in the LiPF6 based electrolyte helps to obtain high-performance batteries (Fig. 1a).
Tms-on has an n-Si structure, which contains a polar ring structure that can effectively interact with Li +, so as to improve the dissociation degree of LiPF6 and reduce the formation of LiPF6 by ions. The Si atom in the n-Si structure of tms-on can effectively absorb fluorine ions to form a pentavalent silane intermediate, and then form a fluorinated trimethylsilyl (TMSF). Therefore, the decomposition of tms-on into TMSF and on may eliminate HF in LiPF6 based electrolyte (Fig. 1b).
Figure 1 the dissociation of LiPF6 and the mechanism of tms-on scavenging HF and H2O
In order to analyze the mechanism of tms-on action on LiPF6 in electrolyte, the DFT calculation was carried out. Firstly, the formation energy of the interaction between electrolyte components and lithium ion is studied. The formation energy of the complex of lithium ion with EC, EMC, Dec, VC, on and tms-on is compared. It is found that tms-on-li + complex has a low minimum formation energy of - 0.81ev, which indicates that tms-on-li + complex has a relatively stable structure. According to the stable coordination of the complex between lithium ion and the solvent or additive in the electrolyte. DFT calculation shows that carbonate molecules (EC, EMC, Dec and VC), on and tms-on have stable effects on PF5 (Fig. 2a). According to the binding energy of PF5, PF5 preferentially coordinated with tms-on and on (1.28 and - 0.73 EV, respectively). This is because the N atoms in tms-on and on are Lewis bases, which can effectively interact with Lewis acid PF5. In addition, the binding distance between the n-position of tms-on (1.95?) and PF5 is shorter than that of on (2.31?), indicating that tms-on can stabilize PF5 more effectively due to the electron donating property of TMS functional group. In order to confirm the effect of PF5 stability on the hydrolysis of LiPF6, the author calculated the reaction between PF5 and H2O with or without tms-on-li + complex (Fig. 2b). In the absence of tms-on-li + complex, PF5 is easier to hydrolyze, while in the presence of tms-on-li + complex, PF5 has a higher energy barrier. Because of the strong interaction between PF5 and tms-on-li + complex, when tms-on exists, the potential barrier of adsorbing H2O on PF5 is higher. On the contrary, when tms-on does not exist, the adsorption barrier of PF5 is low.
Fig. 2 Effect of tms-on additive in electrolyte on the stability of LiPF6
In order to elucidate the working mechanism of tms-on additives in electrolyte, 19F NMR was used to characterize LiPF6. LiPF6 was dissolved in EC, EMC and dec without additives. 0.5 wt% tms-on electrolyte and 0.5 wt% tms-on electrolyte were placed at 45 ℃ for 40 hours. Although pof3 was detected in an additive free electrolyte and an electrolyte containing 0.5 wt% on, it was not detected in an electrolyte containing 0.5 wt% tms-on (Fig. 3a). Based on this, the author speculates that PF5 may react with micro amount of water in the electrolyte without additives and convert into pof3. According to the amount of hpo2f2 and HF produced, the hydrolysis degree of LiPF6 can be inferred. In the electrolyte without additives, the hydrolysis degree of LiPF6 is the largest, followed by the electrolyte containing 0.5wt% on, and the hydrolysis degree of the electrolyte containing 0.5wt% tms-on is the smallest (Fig. 3b-c). In the electrolyte without additives, the concentration of HF continues to increase, while in the electrolyte containing 0.5 wt% tms-on, HF disappears completely, because tms-on reacts with HF to form TMSF over time (Fig. 3D). The concentration of HF and hpo2f2 in the electrolyte will affect the interface structure of graphite anode (Fig. 3G). In order to investigate the influence of tms-on on the structure of SEI, the authors carried out in-situ XPS test, and found that the f 1s spectrum showed significant difference between the graphite anode circulating in the electrolyte without additives and the electrolyte containing 0.5wt% tms-on (Fig. 3e-f). In the electrolyte without additives, the graphite anode after three charge discharge cycles was detected to contain LIF, poxfy and P-F species (Fig. 3e).
Fig. 3 19F NMR analysis of decomposition product concentration of three kinds of electrolytes based on LiPF6 after 40 h at 45 ℃
The molecular energy levels of solvents (EC, EMC and DEC), additives (VC and tms-on) and intermediates (on and TMSF) interacting with Li + in the electrolyte are shown in FIG. 4A. Tms-on is easy to react with HF and decompose into on and TMSF. Compared with the lowest unoccupied molecular orbital (LUMO) energy, VC has the lowest LUMO of - 1.14ev, which may be the most favorable for the reduction and decomposition of graphite anode. Compared with carbonate solvents (EC, Dec and EMC), tms-on, on and TMSF have higher LUMO energy, which suggests that these compounds are unlikely to undergo reductive decomposition at graphite anode. However, tms-on, on, TMSF and VC are likely to undergo oxidative decomposition on the NCM anode because their highest occupied molecular orbital (HOMO) energy is higher than that of carbonate solvent. Figure 4b-e shows the cycle performance (0.5c) of NCM / graphite battery at 25 ℃ and 45 ℃ respectively in the electrolyte with and without 0.5wt% tms-on additive. The cycle stability of NCM / graphite battery with 0.5wt% tms-on is better at 25 ℃ and 45 ℃. When tms-on additive is contained in the electrolyte, the retention rate of discharge capacity increases from 52.4% to 80.4% after 400 cycles at 45 ℃, the discharge capacity is 154.7 Ma h g-1, and the coulomb efficiency is 99.8%. In addition, when the electrolyte contains 0.5 wt% tms-on additive, the ohmic polarization of the whole battery is significantly reduced compared with the case without tms-on additive in the electrolyte (Fig. 4d-e). After 400 cycles at 45 ℃, it can be seen that NCM / graphite full cell with electrolyte containing 0.5 wt% tms-on additive has lower SEI film (RF) impedance and charge transfer impedance (RCT) (Fig. 4F).
Fig. 4 energy level diagram of the interaction between various solvents and additives and lithium ion and the effect of 0.5 wt% tms-on additive in electrolyte on the electrochemical performance of NCM / graphite full cell
X-ray photoelectron spectroscopy (XPS) was used to study the effect of 0.5wt% tms-on additive on the interface structure of NCM anode. The C 1s spectrum in Fig. 5A shows that when there is no tms-on additive in the electrolyte, there is a poly (VC) species produced by VC polymerization on the surface of NCM anode after cycling, because VC has the highest HOMO energy level, it is easy to be oxidized at NCM anode. On the contrary, in the electrolyte containing 0.5 wt% tms-on, no poly (VC) peak was observed at 290.5 EV, while at 286 and 287.4 EV, there were n – C and o = C – n peaks formed by on oxidation decomposition (Fig. 5b). In addition, the appearance of n1s indicates that the positive SEI contains n – C and o = C – n (Fig. 5e). The effect of tms-on additive on HF removal slightly reduced the relative content of LIF and nif2 in SEI (Fig. 5c-d). In the electrolyte without tms-on additive, SEI of NCM anode is mainly composed of VC derivatives. When tms-on additive is contained in the electrolyte, the interface structure of NCM anode is greatly changed by tms-on additive. The dissolution degree of transition metals in NCM positive electrode was characterized by ICP-OES. The NCM positive electrode of SEI formed by VC derivative and the NCM positive electrode of SEI formed by on derivative were stored in the solution without additives at 45 ° C for 7 days. The effect of SEI formed by on derivative on the interface stability of NCM positive electrode is shown in Figure 5F- The SEI formed by the derivatives can inhibit the dissolution of the transition metal in the NCM positive electrode, which indicates that the SEI formed by the on derivatives can improve the interface stability of the NCM positive electrode at 45 ℃. Based on this, the author proposes the mechanism of polymerization of VC and on oxidation radicals, which is initiated by the radical cation (· o – ch = ch +) produced by 1E oxidation VC (Fig. 5g). SEI formed by on derivatives may increase o-ch = ch + attack. Because the HOMO energy level of on is lower than VC, the active o-ch = ch + can attack the carbon between on nitrogen and oxygen, forming a polymer species with amide group.
Fig. 5 XPS analysis of the change of interface structure of NCM anode material after three cycles of NCM / graphite battery at 25 ℃ in different electrolytes
The effect of on on on the interface structure of NCM anode was analyzed by TOF-SIMS. After 400 cycles at 45 ° C, a strong CN signal appears in the TOF-SIMS spectrum containing 0.5 wt% tms-on electrolyte (Fig. 6a). When there is no tms-on additive in the electrolyte, the positive electrode of NCM forms SEI derived from VC, and the signal peaks corresponding to 7lif2 -, PO2 -, and PO3 - appear in TOF-SIMS spectrum. The results show that VC derived SEI is not enough to inhibit the continuous oxidative decomposition of NCM positive electrolyte. mutually