Li-rich manganese-based oxides (LRMO) with high capacities are attractive cathode materials for next-generation lithium-ion batteries. However, poor cycling stability is one of the key issues impeding their commercialization. Here, for the first time, we employed trimethoxy(3,3,3-trifluoropropyl)silane (TMTFS) as a multifunctional electrolyte additive to stabilize the LRMO cathode interphase and elevate its cycling performance. The LRMO electrode delivered a high reversible capacity of 250.4 mAh·g−1 with a stable capacity retention of 91% after 200 cycles. Detailed analysis using in situ powder X-ray diffraction (PXRD), cyclic voltammetry (CV), nuclear magnetic resonance (NMR), X-ray photoelectron energy spectra (XPS) and transmission electron microscopy (TEM) demonstrated that the TMTFS additive can not only form a robust, thin, and dense LiF-dominated cathode electrolyte interphase, but also scavenge detrimental HF in electrolyte. Additionally, the TMTFS additive can adjust the solvation environment, thus enhancing the transference number of lithium ions.

With the explosively growing demand of portable electronic devices and pure/hybrid electric vehicles, it is urgent to develop rechargeable lithium-ion batteries (LIBs) with high energy and high power densities [1]. Compared to anode materials, the energy density of commercial LIBs has reached a bottleneck, which is limited by the cathode materials [2,3,4,5]. The mainly commercial cathode materials have lower capacities, such as LiCoO2 (140 mAh·g−1) [6] and LiFePO4 (150 mAh·g−1) [7, 8]. Therefore, it is crucial to seek or optimize cathode materials with high capacity for next-generation LIBs.

Among the high capacity cathode materials, Li-rich manganese-based oxides (LRMO), a formula of xLiMO2·(1−x)Li2MnO3 or Li1+xM1−xO2 (0 < x < 1, M = Ni, Co, Mn), have attracted considerable attention on high theoretical capacities (> 300 mAh·g−1) and a wide voltage range of 2.0–4.8 V (vs. Li/Li+), compared with Ni-rich NCM (lithium nickel cobalt manganese oxide) and NCA (lithium nickel cobalt aluminum oxide) [9,10,11]. However, the LRMO materials usually have a large initial capacity loss [12, 13], poor rate capability [14, 15], fast capacity decay and continuous voltage fading upon cycling [16,17,18], which seriously hinders them for the practical LIBs applications. Various studies demonstrated that the poor electrochemical performance of LRMO materials originates from the following: (1) the irreversible oxidation of O2− occurred once charged to 4.4–4.8 V (vs. Li/Li+), resulting in a low initial Coulombic efficiency (ICE) [12, 18]; (2) the irreversible oxygen release induced oxygen vacancy and transition-metal ions migration into Li layer, leading to voltage fading and capacity degrading [19,20,21]; (3) the continuous high voltage interfacial side reactions and the dissolution of transition-metal ions in electrolyte under the corrosion of hydrofluoric acid, resulting in thick cathode electrolyte interphase (CEI) layer and poor rate capability [22, 23], respectively.

To solve/mitigate the above detrimental effects, several strategies have been reported. One strategy focuses on the cathode materials, such as surface modification [24, 25], cation or anion doping [14, 26, 27]. One typical example is coating Li3PO4 with high ionic conductivity and the substances with high electronic conductivity on the surface of high-capacity layered oxide cathode materials, resulting in restraining the interfacial side reactions, and promoting the ionic and electronic conductivity [24, 28,29,30,31]. Recently, Wynn and co-workers [32] demonstrated that the Mo-doped LRMO tuned charge density distribution and the local band structure to enhance the electrochemical performances. Another strategy focuses on the electrode construction, i.e., introducing functional binder or conductivity to build the electrode [33, 34]. Various binders, such as sodium carboxymethyl cellulose (CMC) and polyacrylic acid (PAA), were employed to construct the stable electrodes, which indicated that the voltage-fading phenomena were alleviated [17, 35]. The third strategy focuses on the electrolyte, for instance, adding new solvents or additives [36,37,38]. Meng and co-workers used LiBOB as an additive, which can remove HF and form stable CEI layer, resulting in relieving particles’ surface/subsurface phase transformation and alleviating voltage decay [39]. Similar study was also carried out by Zhang et al. [23]. Compared to the above three strategies, adding electrolyte additive is an easier approach, which can be accepted by the battery manufacturers.

In this study, we are the first to choose TMTFS as a functional electrolyte additive. The electrochemical performance of this additive displayed higher cycling stability of LRMO electrodes. In situ PXRD, NMR, XPS and TEM revealed the TMTFS additive to form a thin and uniform LiF-enriched CEI layer, which effectively protected the LRMO particles during the cycling. The TMTFS additive tuned the solvation environment of lithium ions, resulting in enhancing Li-ion transference number and inhibiting the decompose of PF6−. Meanwhile, it can also effectively scavenge hydrogen fluoride (HF), which can corrode the LRMO structure, from the electrolyte.

The LRMO (Li1.2Ni0.13Co0.13Mn0.54O2) material was synthesized by a simple polymer-pyrolysis method [17, 40,41,42]. The structure and morphology of the as-synthesized LRMO were characterized by PXRD, SEM and TEM, as shown in Fig. 1. PXRD pattern of the LRMO has a typical layered hexagonal structure of α-NaFeO2 lattice with space group of R\(\bar{3}\)m (Fig. 1a). Meanwhile, the peaks from 20° to 25° belonged to the formation of Li2MnO3, which were referred to as superlattice reflections [43]. It was worth noting that the clearly split diffraction peaks of (006), (012), (018) and (110) were observed, indicating the sample with a well-layered structure. The ratio of the peak intensity of (003)/(104) in the PXRD pattern corresponds to the cation disorder degree between lithium and transition metal sites [24, 40]. In addition, the particle size of LRMO nanoparticle is about 200–300 nm (Fig. 1b, c). The clear edge and angle were observed in each of particles from SEM image, which confirmed the LRMO nanoparticles with crystal structure. High-resolution transmission electron microscopy (HRTEM) image (Fig. 1d) indicates that the distance between adjacent lattice plane is measured about 0.47 nm, which is in good agreement with the d-spacing of the (003) plane of the LRMO.

Fig. 1
figure 1

a XRD pattern, b SEM image, c TEM image and d HRTEM image of synthesized Li1.2Ni0.13Co0.13Mn0.54O2 nanocrystals

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Coin cells were employed to evaluate electrochemical properties of the LRMO cathode in the LB-372 electrolyte with and without TMTFS additive. Figure 2a presents the initial charge–discharge profiles of LRMO electrodes in LB-372 and LB-372-1.0 vol% TMTFS electrolytes. Both of them exhibit similar charge–discharge behaviors with a considerable discharge capacity of 286.3 and 283.3 mAh·g−1 for LB-372 and LB-372-1.0 vol% TMTFS electrolytes, respectively. Figures 2b and S1b show the cycle performance of batteries with different electrolytes at 0.4C (1.0C = 250 mA·g−1). LRMO electrode in the LB-372 electrolyte displayed an initial capacity of 241.1 mAh·g−1 and the capacity retention of 77% after 200 cycles. While the LRMO cathode in LB-372-1.0 vol% TMTFS electrolyte showed clearly improved stability, which had the first capacity of 250.4 mAh·g−1, corresponding capacity retention of 91% under identical conditions. When the content of TMTFS additive increased to 2.5 vol% and 5.0 vol% (Fig. S2a), the capacity retention was 89% and 83% after 200 cycles at 0.4C. Additionally, different fluorosilane and n-propyltrimethoxysilane (without fluorine) were chosen as control experiments, as shown in Fig. S2c, d, which demonstrated the TMTFS additive with the best electrochemical performance among them. To further assess the stability at high current densities, we measured the rate dependence of specific capacity of the LRMO cathode in the electrolyte with and without 1.0 vol% TMTFS, as shown in Figs. 2c, S1a–d. When the electrolyte included TMFTFS, the reversible capacities of the electrodes are 195.8 and 162.2 mAh·g−1 after 200 cycles at 1.0C and 2.0C, respectively. An over-charge phenomenon was observed in the LB-372 electrolyte after over 100 cycles in all current densities (Fig. S1b–d). This phenomenon brought forward to ~ 40th cycle when the charge voltage range extended to 5.0 V (Figs. 2d, S1e). This result illustrated that the LB-372 electrolyte is unstable for the high voltage cathode materials during cycling. Fortunately, the TMTFS additive can improve the stability of electrolyte at high voltage during charge process and overcome this over-charge phenomenon.

Fig. 2
figure 2

a Initial charge–discharge profiles of LRMO electrode at 25 mA·g−1 in LB-372 and LB-372-1.0 vol% TMTFS electrolytes; cycling performance of LRMO cathodes at a current density of b 100 and c 500 mA·g−1 in LB-372 and LB-372-1.0 vol% TMTFS electrolytes between 2.0 and 4.8 V; d cycling performance of LRMO electrode at a current density of 100 mA·g−1 in LB-372 and LB-372-1.0 vol% TMTFS electrolytes between 2.0 and 5.0 V; e 7Li NMR spectra of LB-372 and LB-372-1.0 vol% TMTFS electrolytes; f 19F NMR spectra of LB-372 electrolyte and with HF or/and TMTFS

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To understand why the electrolyte with TMTFS additive exhibits excellent cycling performance, we carried out analysis on the electrolytes and the electrode/electrolyte interface. We first performed NMR measurements on electrolyte with and without TMTFS additive to study the chemical environment of Li and F. When the TMTFS was added into the electrolyte, it could change the solvation structure of Li+. Upon adding the additive in the LB-372 electrolyte, the 7Li peak shifts downfield (more positive, Fig. 2e), which is indicative of decreased Li+-solvent interaction, resulting in less electron density around Li+ [40]. It means that the less solvent molecules are around Li+, which can move faster in this electrolyte during charge–discharge processes. This result was confirmed by the lithium-ion transference number through the Bruce–Vincent–Evans technique (Fig. S3) [44].

The solute of electrolyte, LiPF6, is a moisture-sensitive component, which is easy decomposed to form HF and PO2F2−. HF is a harmful species, resulting in etching the LRMO and current collectors [45, 46]. Therefore, the electrolyte with the function of scavenging HF is one of the strategies to enhance the cycling performance. The 19F NMR spectra of electrolytes are shown in Figs. 2f, S4. When adding HF aqueous solution in LB-372 electrolyte, new peaks belonging to HF (− 192 × 10–6) and PO2F2− (− 85 × 10–6 and − 87 × 10–6) were observed. The 19F peak of HF was not detected after adding HF in LB-372 electrolyte with 1.0 vol% TMTFS. Meanwhile, the peak intensity of PO2F2− clearly decreased. It demonstrated that the TMTFS additive not only scavenged HF, but also suppressed the hydrolysis of LiPF6. This result is different with that of (2-allylphenoxy) trimethylsilane additive, which still observed peaks of PO2F2− [47].

Both of the 7Li and 19F NMR spectra revealed that the TMTFS additive can change the structure of electrolyte, and it also will change the interface between cathode and electrolyte. HRTEM, XPS, electrochemical impedance spectroscopy (EIS) and CV were employed to analyze and characterize the electrode/electrolyte interface. Firstly, the LRMO particles after charge–discharge cycles in both electrolytes were characterized through TEM, as shown in Fig. 3a, b. A roughness layer, with thickness from nanometers to 25 nm, was observed on the LRMO particle in LB-372 electrolyte. It seems to be etched by HF, which is from hydrolysis of LiPF6 or the decomposition of the electrolyte under high potential. Such thick and unshaped interphase layer may hinder the transportation of Li+ between electrolyte and LRMO particles. In contrast, the surface of the LRMO particles has a uniform and smooth layer when adding TMTFS into the electrolyte. The thickness of the interphase layer is about 4 nm. Compared to both TEM images (Fig. 3a, b), they confirmed that the TMTFS affected the electrode/electrolyte interface.

Fig. 3
figure 3

TEM images of LRMO electrodes after cycling in a LB-372 and b LB-372-1.0 vol% TMTFS electrolytes; XPS spectra of LRMO electrodes after cycling in LB-372 (c and e) and LB-372-1.0 vol% TMTFS (d and f) electrolytes

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To elucidate the chemical component of the electrode/electrolyte interface, i.e., CEI layer, XPS was used to check the chemical bonding environments of the species. Figures 3c–f and S5 display the deconvolution of C 1s, F 1s and Si 2p XPS spectra of the samples after charge–discharge cycles. The deconvolution of the C 1s spectrum indicated that peaks of RO–CO2Li/Li2CO3 (290.8 eV), C=O (287.9 eV), and C–O (286.2 eV) were attributed to the decomposed of solvent or additives [37, 47]. Compared to the reference peak (284.8 eV), the intensity of those three peaks is lower in the electrolyte with TMTFS than that of the electrolyte without additive (Fig. 3c, d). This result illustrated that this additive alleviated solvent decomposition. The F 1s XPS spectra indicated three species, i.e., C–F (687.9 eV), LixPOyFz (687.2 eV) and LiF (685.3 eV), as shown in Fig. 3e, f. The intensity of the peak of LixPOyFz is clearly decreased, but the peak of LiF is significantly increased when this additive is added into electrolyte. Combined to the deconvolution of the Si 2p spectrum (Fig. S5), it revealed that some of TMTFS was decomposed to construct the electrode/electrolyte interface, and formed a robust, thin, and dense LiF-dominated CEI layer, which is further confirmed by TEM images (Fig. 3) and EIS results (Fig. S6 and Table S1). Furthermore, SEM images showed that LRMO materials in LB-372-1.0 vol% TMTFS electrolyte contacted more tightly after 200 cycles (Fig. S7). It was worth noting that the clearly split diffraction peaks of (006), (012), (018) and (110) still existed in LB-372-1.0 vol% TMTFS electrolyte after cycles (Fig. S8). These results are believed that the TMTFS additive is to be the key factor in achieving the excellent cycling performance of LRMO electrode due to forming a unique CEI layer.

CV was employed to further assess the electrochemical performance of the electrode/electrolyte interface. The CV with various scan rates was recorded, as shown in Figs. 4, S9. When the scan rate increased from 0.1 to 0.5 mV·s−1, the anodic peak (A) shifted by 212 and 52 mV to higher potentials, while the cathodic peak (B) shifted by 300 and 160 mV to lower potentials for electrolyte without/with the TMTFS additive, respectively. Obviously, there is a larger polarization in electrolyte without the additive, which can be further supported by TEM images (Fig. 3). We calculated the apparent lithium-ion diffusion coefficients, which reflects the kinetics of lithium insertion/extraction at the electrode/electrolyte interface (Fig. 4e, f), according to the classical Randles–Sevcik equation [48]. Clearly, all the apparent lithium-ion diffusion coefficients of the battery with the TMTFS additive are larger than that of battery without this additive (Table S2). This difference value of the diffusion rates and polarization can be attributed to the structure and component of the electrode/electrolyte interface, which was improved by the TMTFS additive. This result is further supported by the excellent cycling performance of the LRMO electrodes with the TMTFS additive.

Fig. 4
figure 4

CV curves of LRMO electrodes with different scanning rates: 0.1–0.5 mV·s−1 in a LB-372 and b LB-372-1.0 vol% TMTFS electrolytes, and corresponding relationship between c, d Vp and v, e, f Ip and v1/2

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We carried out in situ PXRD studies of the LRMO electrode to check the structure changes during the first three cycles under the current density of 25 mA·g−1. The contour map of in situ PXRD and the corresponding charge–discharge profiles are displayed in Figs. 5, S10. The unshifted peaks are attributable to current collector, beryllium window, and the device of the in situ cell. During the first charging process, some peaks, including (003) and (018), shift to lower angle at first, then keep a period of time, and then a little back to higher angle. It is attributed that lithium ions are extracted from lithium layers and transition metal layers with different structure changes, resulting in c-parameter increasing at first, then keeping constant, and then decreasing later [49]. With extraction of lithium from the lithium layers, the electrostatic repulsion between oxygen layers induced the increase of c-parameter. With further extraction of lithium to ~ 4.5 V, the lithium of transition metal layers was activated and entered into the lithium layers, which kept the c-parameter constant during this process. When the speed of extraction lithium is larger than that of lithium from transition metal layers entered into lithium layers, the lithium lays contracted, meanwhile, oxygen released and transition metal entered into the lithium layers, which may decrease the electrostatic repulsion and then decrease the c-parameter [49]. Other peaks shifted to higher angle, which is related to a-parameter [50]. During the initial discharge process, the lithium ions also have two processes, i.e., insertion into lithium layers and transition metal layers, resulting in the peaks shifted to inverse direction of the charging process. During subsequent two cycles, similar phenomena were observed. The periodic shift of the peaks indicated a reversible lithiation/delithiation process. It should be noted that the positions of (003) and (018) peaks at full charge and full discharge stages are almost constant, which is related to the change of c-parameter. This phenomenon observed by in situ PXRD is similar to previous reports [49,50,51].

Fig. 5
figure 5

a, b First three-cycles curves and corresponding in situ PXRD patterns of LRMO electrode in LB-372-1.0 vol% TMTFS electrolyte at 25 mA·g−1; c discrete PXRD patterns selected from Panel a

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In summary, we report TMTFS as a new multifunctional electrolyte additive, which significantly enhance the cycling stability of the Li1.2Ni0.13Co0.13Mn0.54O2 cathode. A LRMO electrode delivered a reversible capacity about 227.6 and 195.8 mAh·g−1 at 0.4C and 1.0C after 200 cycles with a capacity retention of 91% and 81%, respectively, compared to those (only 77% and 46%) in electrolyte without TMTFS under the same conditions. 7Li and 19F NMR spectra revealed that the TMTFS additive can change the structure of electrolyte, the solvation environment of lithium ions and HF scavenging. XPS, TEM, EIS and CV results confirmed that the TMTFS additive can promote a robust, thin, and dense LiF-dominated cathode electrolyte interphase on the LRMO cathode. From this study, a fundamental understanding of the function of TMTFS additive can serve as a guide for designing new additives for Li-rich layered oxide cathode materials.