Single-ion conducting polymer electrolyte for Li||LiNi0.6Mn0.2Co0.2O2 batteries—impact of the anodic cutoff voltage and ambient temperature

Polymer-based electrolytes potentially enable enhanced safety and increased energy density of lithium-metal batteries employing high capacity, transition metal oxide–positive electrodes. Herein, we report the investigation of lithium-metal battery cells comprising Li[Ni0.6Mn0.2Co0.2]O2 as active material for the positive electrode and a poly(arylene ether sulfone)-based single-ion conductor as the electrolyte incorporating ethylene carbonate (EC) as selectively coordinating molecular transporter. The resulting lithium-metal battery cells provide very stable cycling for more than 300 cycles accompanied by excellent average Coulombic efficiency (99.95%) at an anodic cutoff potential of 4.2 V. To further increase the achievable energy density, the stepwise increase to 4.3 V and 4.4 V is herein investigated, highlighting that the polymer electrolyte offers comparable cycling stability, at least, as common liquid organic electrolytes. Moreover, the impact of temperature and the EC content on the rate capability is evaluated, showing that the cells with a higher EC content offer a capacity retention at 2C rate equal to 61% of the capacity recorded at 0.05 C at 60 °C.


Introduction
Rechargeable lithium-metal batteries are considered the next great leap forward towards higher energy densities [1].Nevertheless, the severe risk of lithium dendrite formation, potentially causing a short circuit of the cell, and the continuous electrolyte decomposition at the electrode|electrolyte interface have so far hampered the commercial exploitation of such batteries -with one little exception: lithium-polymer batteries comprising an electrolyte based on poly(ethylene oxide) (PEO) [2].This polymer electrolyte, however, suffers of two major drawbacks related with the operating temperature of about 60-80 °C (especially during charge when Li plating occurs) and the choice of the active material for the positive electrode is limited to materials which are de-/lithiated below 4 V, such as LiFePO4 [2][3][4][5][6].These two issues originate from the facts that the charge transport is coupled with the segmental motion of the polymer, which is faster the higher the temperature, and that the lithium-coordinating ether group and/or terminal hydroxyl groups are not sufficiently stable towards oxidation beyond 4 V. [4,[7][8][9][10][11] Another issue is the potential reversed cell polarization at elevated current densities, leading to accelerated cell failure and favoring inhomogeneous (dendritic) lithium deposition [12][13][14][15].
Very recently, we have reported a new multi-block co-poly(arylene ether sulfone) electrolyte which addresses these issues by covalently tethering the anionic function to the polymer backbone, stabilizing the ether group through adjacent electron-withdrawing groups, and introducing, e.g., ethylene carbonate (EC) as "molecular transporter" to facilitate Li + conduction; the latter effect being facilitated by the selective coordination of EC to the ionophilic block (psi-PES) in which the charge transport occurs, while the ionophobic blocks (FPES) provide mechanical stability [16,17].This nanophase-separated, single-ion conducting, multi-block copolymer electrolyte comprising EC allows for the stable cycling of Li||Li[Ni1/3Mn1/3Co1/3]O2 (NMC111) full-cells with an anodic cut-off voltage of 4.2 V for more than 200 cycles at 40 °C with a specific capacity of about 100 mAh g -1 at C/5 (32 mA g -1 ) [16,17].
Herein, we investigate the use of this multi-block copolymer electrolyte in high-energy Li||Li[Ni0.6Mn0.2Co0.2]O2(NMC622) cells, as schematized in Figure 1, with particular regard to the impact of the stepwise increasing anodic cut-off voltage from 4.2 V to 4.3 V and 4.4 V.Moreover, we investigated the effect of increasing the EC concentration and the ambient temperature on the achievable rate capability, revealing very good rate capability thanks to an ionic conductivity well above 1 mS cm -1 under such conditions.

Copolymer synthesis
The synthesis of the partially fluorinated multi-block poly(arylene ether sulfone) with covalently tethered lithium perfluorosulfonimide functions (herein referred to as SI), serving as electrolyte, and its characterization have been described in detail in Nguyen et al. [16] Briefly, the block copolymer backbone (with block lengths 15 kg/mole) was synthesized via co-polycondensation and subsequently region-selectively brominated in order to allow for the covalent tethering of the sulfonimide anion using Ullman's coupling reaction [18] and lithium precursor.EC was purchased from Merck or BASF and used as received.The copolymer was characterized via 1 H NMR and 19 F NMR spectroscopy to confirm the molecular architecture.
Size-exclusion chromatography coupled with multiangles laser light scattering dectecteur revealed a weigh average molecular weight (Mw) of 724 kg mol -1 .The solvent content (SC), indicated as x in SIx%, is defined as ration between the mass of the EC swollen membrane (Ms) and the sum of the of Ms and the dried membrane (Md) multiplied by 100% [17]: The handling and processing of the ionomer membranes were conducted either in an argonfilled glove box or in the dry room to avoid a relevant impact of moisture.

Electrode preparation and electrochemical characterization
The NMC622 electrodes preparation was performed in the dry room as well.NMC622 (commercial source) was dispersed together with Super C65 (IMERYS) and poly(vinylidene difluoride) (PVdF, Solvay) in N-methyl-2-pyrrolidone (NMP, Aldrich) via magnetic stirring (3 h at 500 rpm).The resulting slurry was cast on aluminum foil using a laboratory-scale doctor blade with a wet film thickness of 130 µm.The resulting electrode sheets were dried at 60 °C overnight.Disk-shaped electrodes ( = 12 mm) were punched from the thus pre-dried electrode sheets and subsequently dried at 120 °C for 12 h under vacuum.Eventually, the electrodes were pressed at 10 t for 10 s.The total electrode composition was 88 wt% NMC622, 7 wt% Super C65, and 5 wt% PVdF.The active material mass loading was around 2.0 ±0.2 mg cm -2 .For the electrochemical characterization, two-electrode, Swagelok-type cells were assembled sandwiching the ionomer electrolyte membrane between the NMC622 electrode and the lithium metal electrode (Honjo, battery grade).The whole cell was subjected to a pressure (4-5 t) for 3 min to infiltrate the ionomer electrolyte into the porous NMC622 electrode.Galvanostatic cycling was conducted using a Maccor 4000 battery tester.The temperature was controlled by placing the cells in climatic chambers (Binder).The cathodic cut-off voltage was kept constant at 2.8 V throughout all tests reported herein, while the anodic cut-off voltage was varied from 4.2 V to 4.3 V and 4.4 V.A dis-/charge rate of 1C corresponds to a specific current of 160 mA g -1 .For comparison, also cells with a liquid organic electrolyte (1M LiPF6 in EC-DMC, 1:1 by weight; UBE) were assembled and tested.For the determination of the limiting current density, two-electrode pouch cells were assembled with the ionomer electrolyte membranes sandwiched between two lithium foils.The cells were kept in a climatic chamber at the corresponding temperature for 6 h prior to the application of a sweep rate of 0.025 mV s -1 using a Solatron 1400 CellTest system.

Results and discussion
To start with, Li|SI55%|NMC622 cells were subjected to galvanostatic cycling at 40 °C, setting the anodic cut-off voltage to 4.2 V in order to have a direct comparison with the results reported earlier for Li|SI55%|NMC111 cells [16].Figure 2a shows the dis-/charge profiles for the initial five formation cycles at 0.05C.The cell delivers a specific capacity of 157 mAh g -1 , while the first cycle Coulombic efficiency is about 85%.The subsequent long-term cycling (300 cycles performed at 0.5C) is presented in Figure 2b.The delivered capacity during the first cycles is 93 mAh g -1 , which slightly decreased upon cycling to stabilize at about 85 mAh g -1 .As a result, the capacity retention was 91.1%, 87.2%, and 81.1% after 100, 200, and 300 cycles, respectively.This impressive cycling stability considering the use of the Li metal electrode is accompanied by a very high average Coulombic efficiency of 99.95%, which is a remarkably high as a result of the excellent compatibility of the ionomer electrolyte with Li metal.
In a next step, the cells were tested using higher upper cut-off voltage (4.3 V and 4.4 V, see

Figure 3a
).The dis-/charge rate was initially varied from 0.05C up to 2C to investigate the rate performance and then kept constant at 0.5C in order to study the cycling stability.At 0.05C, the reversible specific capacity, i.e., the capacity obtained upon lithiation (discharge), increases from the 157 mAh g -1 (@ 4.2 V) to 162 and 173 mAh g -1 when elevating the anodic cut-off voltage to 4.3 and 4.4 V, respectively.This trend is maintained increasing the C rate to 0.1C with slightly higher capacities for the Li|SI55%|NMC622 cycled with an anodic cut-off of 4.4 V. When further increasing the dis-/charge rate to 0.2C, however, very similar capacities were obtained.Finally, at even higher C rates, the cells cycled with an anodic cut-off of 4.3 V showed higher capacities than the cell cycled with an anodic cut-off of 4.4 V. Precisely, specific discharge capacities of about 108, 86, and 49 mAh g -1 were obtained at 0.5C, 1C, and 2C, respectively, for an anodic cut-off of 4.3 V, while setting the anodic cut-off to 4.4 V led to capacities of around 103, 78, and 36 mAh g -1 at 0.5C, 1C, and 2C, respectively.After this rate capability test, the dis-/charge rate was set constantly to 0.5C to evaluate the general cycling stability.For an anodic cut-off of 4.3 V, the cells provided very stable cycling with about 73.8% capacity retention after 200 cycles (i.e., about 80 mAh g -1 ).A very similar capacity retention of 74.4% was obtained for an anodic cut-off of 4.4 V, but after 100 cycles.These results suggest that the detrimental reactions occurring at the interface between the cathode and the ionomer electrolyte are more pronounced when elevating the anodic cut-off voltage to 4.4 V and that this effect outweighs the initially beneficial impact on the achievable specific capacity.This is in good agreement with a recent study on a very similar ionomer electrolyte system [17].
74.0%, as depicted in Figure 3b -despite the generally higher specific capacity across all dis-/charge rates, which is assigned to the substantially higher ionic conductivity (>10 mS cm −1 at 20 °C10 vs. <1 mS cm −1 at 40 °C [16]) and potentially a better penetration of the electrolyte in the electrode's pores.This result indicates that the fading is, at least partially, related to the performance of the cathode active material itself when setting the upper cut-off to 4.4 V and/or that the ionomer electrolyte offers the same oxidation stability of organic carbonate-based liquid electrolytes, in spite of the reportedly poor stability of ethylene carbonate at elevated potentials [19].
Following these results, 4.3 V was chosen as the anodic cut-off voltage for the subsequent investigation of the impact of the EC content and ambient temperature on the rate capability of Li|SIx%|NMC622 cells (Figure 4).In Figure 4a, the evaluation of the rate capability for Li|SI55%|NMC622 at 40 °C is shown again, serving as reference.The same test was subsequently applied for Li|SI65%|NMC622 cells, i.e., employing a higher EC content in the ionomer membrane (Figure 4b).This increase in EC concentration leads to higher capacities across all dis-/charge rates, particularly, at C rates above 0.5C.At 1C, for instance, the capacity increased from about 86 mAh g -1 to 102 mAh g -1 , and at 2C the capacity increased from around 49 mAh g -1 to 78 mAh g -1 , corresponding to relative increases of about 19% and 58% at 1C and 2C, respectively.This superior rate capability is assigned to the improved ionic conductivity for SI65% compared to SI55% (around 1.2 mS cm −1 vs. 0.5 mS cm −1 at 40 °C [16]) and the enhanced limiting current density (1.3 mA cm −2 at 40 °C vs. 1.2 mA cm −2 at 50 °C).In fact, when increasing the ambient temperature to 60 °C (Figure 4c), a further rate capability improvement is observed -also at the lowest C rate of 0.05C, resulting in a specific about 171 mAh g -1 vs. 160 mAh g -1 for the Li|SI65%|NMC622 cells.At 2C, the specific capacity was still around 104 mAh g -1 , which translates into a capacity retention of around 61% with regard to the capacity obtained at 0.05C -or an increase in capacity by 34% compared to the Li|SI65%|NMC622 cells run at 40 °C.As a matter of fact, the enhanced kinetics are reflected also in an increased limiting current density with an increased temperature of around 1.5 mA cm −2 (vs.1.3 mA cm −2 at 40 °C).The overall comparison of the rate capability when varying the EC content and elevating the ambient temperature is summarized in Figure 4d, highlighting the stepwise improvement for an increased EC concentration and testing temperature.

Conclusions
Li||NMC622 cells comprising single-ion conducting SIx% as electrolyte provide excellent cycling stability for more than 300 cycles.Elevating the anodic cut-off voltage to 4.3 V and