Wet Mechanical Milling Induced Phase Transition to Cubic Anti-Perovskite Li$_{2}$OHCl

Di-Xing Ni(倪地兴)$^{1\dag}$, Yao-Dong Liu(刘耀东)$^{1\dag}$, Zhi Deng(邓志)$^{1}$, Dian-Cheng Chen(陈典诚)$^{1}$,\\ Xin-Xin Zhang(张欣欣)$^{2}$, Tao Wang(王涛)$^{3}$, Shuai Li(李帅)$^{1\hyperlink{s}{*}}$, and Yu-Sheng Zhao(赵予生)$^{1}$

doi:10.1088/0256-307X/39/2/028201

⇦Chinese Physics Letters, 2022, Vol. 39, No. 2, Article code 028201⇨ Wet Mechanical Milling Induced Phase Transition to Cubic Anti-Perovskite Li$_{2}$OHCl Di-Xing Ni (倪地兴)1†, Yao-Dong Liu (刘耀东)1†, Zhi Deng (邓志)1, Dian-Cheng Chen (陈典诚)1, Xin-Xin Zhang (张欣欣)2, Tao Wang (王涛)3, Shuai Li (李帅)1*, and Yu-Sheng Zhao (赵予生)1 Affiliations 1Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055, China 221C Innovation Laboratory, Contemporary Amperex Technology Ltd. (CATL), Ningde 352100, China 3Guangdong–Hong Kong–Macao Joint Laboratory for Neutron Scattering Science and Technology, Dongguan 523803, China Received 29 October 2021; accepted 22 December 2021; published online 29 January 2022 ^†Di-Xing Ni and Yao-Dong Liu contributed equally to this work.
^*Corresponding author. Email: lis6@sustech.edu.cn Citation Text: Ni D X, Liu Y D, Deng Z et al. 2022 Chin. Phys. Lett. 39 028201 Abstract Anti-perovskite solid-state electrolyte Li$_{2}$OHCl usually exhibits orthorhombic phase and low ionic conductivity at room temperature. However, its ionic conductivity increases greatly when the temperature is up to 40 ℃, while it goes through an orthorhombic-to-cubic phase transition. The cubic Li$_{2}$OHCl with high ionic conductivity is stabilized at room temperature and even lower temperature about 10 ℃ by a simple synthesis method of wet mechanical milling. The cubic Li$_{2}$OHCl prepared by this method performs an ionic conductivity of $4.27 \times 10^{-6}$ S/cm at room temperature, about one order of magnitude higher than that of the orthorhombic Li$_{2}$OHCl. The phase-transition temperature is decreased to around 10 ℃. Moreover, it can still remain cubic phase after heat treatment at 210 ℃. This work delivers a huge potential of fabricating high ionic conductivity phase anti-perovskite solid-state electrolyte materials by wet mechanical milling.

DOI:10.1088/0256-307X/39/2/028201 © 2022 Chinese Physics Society Article Text In recent years, lithium-ion conductors have become a new research hotspot and have attracted extensive attention because they can be used as solid-state electrolytes (SSEs), which play key role in all solid-state rechargeable lithium batteries (ASSLBs) that were expected to have higher safety and larger energy-density than conventional lithium-ion batteries.[1–3] With decade years development, several types of SSEs with high ionic conductivities up to 10$^{-3}$ S/cm have been successfully designed and synthesized, including oxide-based SSEs, halogen-based SSEs and sulfur-based SSEs. However, oxide-based SSEs generally are hard and fragile ceramics synthesized by high-temperature sintering, leading to poor interfacial contact and large interfacial resistance with electrodes. Halogen-based and sulfur-based SSEs generally suffer from narrow electrochemical window and poor air stability.[4–6] The fact that anti-perovskite structural Li$_{2}$OHCl possesses relatively high ionic conductivity and soft nature has attracted broad attention.[7–9] Furthermore, in aspect of stability with Li metal for Li/Li$_{2}$OHCl/Li symmetric cells, Li$_{2}$OHCl is more excellent in contrast to other solid electrolytes.[7] Earlier experimental studies have shown that Li$_{2}$OHCl will undergo phase transition from orthorhombic structure to cubic structure at about 40 ℃, and the ionic conductivity of the former is lower than that of the latter.[10] The calculation results also suggest that Li$_{2}$OHCl with cubic structure has higher ionic conductivity due to strongly collective movement of the migrated Li ions.[11,12] In this work, we use wet mechanical milling methods to obtain the Li$_{2}$OHCl with cubic structure at room temperature, which lets the phase-transition temperature from orthorhombic structure to cubic structure drop down to 10 ℃. Furthermore, the Li$_{2}$OHCl treated by 210 ℃ still maintains cubic phase. Li$_{2}$OHCl used in this work was prepared by wet mechanical milling (WWM). First, weighted 0.05 mol of lithium chloride (LiCl, $> 99.9$%, Aladdin) and 0.055 mol of lithium hydroxide (LiOH, $> 99.9$%, Aladdin) respectively in the glove box and put them into a 30 mL ball milling jar containing 60 g zirconia balls (8 mm in diameter). Then, added 10 mL n-hexane into it and used a planetary ball-mill for continuous wet ball milling at 400 rpm for 10 h. Finally, the suspension after ball milling was dried under vacuum cabin for 12 h to obtain dried white powders, named as Li$_{2}$OHCl-WMM. Moreover, in order to study the effect of annealing on the structure of the samples prepared by this method, the samples were heated up to 210 ℃ at a heating rate of 3 ℃/min and held at 210 ℃ for 5 h, then cooled naturally to room temperature, which is named as Li$_{2}$OHCl-WMM-210 ℃. In contrast, we also prepared Li$_{2}$OHCl by solid-state sintering (SSS). The same raw materials of LiCl and LiOH as the above-mentioned method were placed in a nickel crucible and heated in box furnace at 360 ℃ for 2 h. After the reaction was completed and naturally cooled to room temperature, took out the solidified Li$_{2}$OHCl block and ground it again to obtain the powder, named as Li$_{2}$OHCl-SSS. X-ray diffraction (XRD) with Cu $K_\alpha$ radiation was applied to analyze crystal structure of the prepared samples while the voltage was held at 45 kV and the electric current was held at 200 mA ($2\theta =20$–$90^{\circ}$). The powder should be placed in the groove of the sample table and sealed with Kapton film to isolate moisture in air and prevented the hydrolysis reaction of the sample during transfer and test. Electrochemical impedance spectroscopy (EIS) was carried out to measure ionic conductivity of the Li$_{2}$OHCl. In this study, powders of Li$_{2}$OHCl were pressed into a pellet with the diameter of 10 mm under 360 MPa for 6 min in the glove box at room temperature between $-$10 and 120 ℃ in the frequency range from 1 MHz to 1 Hz with an AC amplitude of 20 mV. The morphology and microstructure of fibers were characterized by scanning electron microscopy (FESEM, HITACHI SU8010), with an accelerating voltage of 200 kV. Figure 1 shows the PXRD pattern of the Li$_{2}$OHCl synthesized by three different methods measured at 25 ℃. For Li$_{2}$OHCl-SSS, it is clear that the diffraction pattern is fitted with an orthorhombic phase (${Pmc}2_{1}$) of Li$_{2}$OHCl whose lattice constants are $a=3.87501$ Å, $b=3.82843$ Å, $c=8.00396$ Å, in agreement with those reported previously.[13,14] For Li$_{2}$OHCl-WMM and Li$_{2}$OHCl-WMM-210 ℃, it is also clear that the two diffraction patterns are in accord with a cubic phase (${Pm}\bar{3}m$) of Li$_{2}$OHCl whose lattice constants are $a=b=c=3.90812$ Å. More importantly, the structure of Li$_{2}$OHCl synthesized by a WWM method has not changed after heat treatment at 210 ℃, which indicates that Li$_{2}$OHCl-WWM have good thermal stability.

Fig. 1. PXRD patterns plot for Li$_{2}$OHCl-SSS, Li$_{2}$OHCl-WMM, and Li$_{2}$OHCl-WMM-210 ℃.

Fig. 2. PXRD patterns at temperatures from 30 to 120 ℃ for (a) Li$_{2}$OHCl-SSS, $-250$ to 25 ℃, (b) Li$_{2}$OHCl-WMM, $-100$ to 25 ℃, and (c) Li$_{2}$OHCl-WMM-210 ℃.

Figure 2 shows the temperature-dependent PXRD patterns of the Li$_{2}$OHCl synthesized by three different methods. For Li$_{2}$OHCl-SSS [Fig. 2(a)], when the temperature changes from 40 ℃ to 50 ℃, a structural phase transition occurs. According to the analysis of the above PXRD pattern, the structure of Li$_{2}$OHCl is orthorhombic phase below 40 ℃ and cubic phase above 50 ℃. It is worth noting that when temperature decreases from high temperature to 30 ℃, the transition occurs from cubic phase to orthorhombic phase. In other words, for Li$_{2}$OHCl synthesized by solid-state sintering, phase transition caused by temperature is reversible. For Li$_{2}$OHCl-WMM [Fig. 2(b)], when the temperature drops from $-150 ^{\circ}\!$C to $-200 ^{\circ}\!$C, peak at 45$^{\circ}$ broadens and splits, and peak at 60$^{\circ}$ shifts to the right, which indicates that the cubic-to-orthorhombic phase transition occurs at low temperature. Furthermore, the peaks at 45$^{\circ}$ and 60$^{\circ}$ return to the same shape when the solid electrolyte is heated from $-250 ^{\circ}\!$C to 25 ℃, which indicates that the phase transition is reversible. For Li$_{2}$OHCl-WMM-210 ℃, it can be observed that the position of diffraction peak at 32$^{\circ}$ does not change upon cooling, but from $-10 ^{\circ}\!$C to $-80 ^{\circ}\!$C, the peaks at 45$^{\circ}$ and 60$^{\circ}$ split and shift to right; while upon heating from $-80 ^{\circ}\!$C to 25 ℃, the peaks cannot return initial shape, proving that the cubic-to-orthorhombic phase transition occurs at low temperature and does not return back with heating. As mentioned above, both PXRD and temperature-dependent PXRD patterns indicate that cubic Li$_{2}$OHCl can be stabilized at room temperature by a WWM method preparation. The PXRD pattern of Li$_{2}$OHCl-WMM also index the cubic phase after heating up to 210 ℃. Upon cooling, both Li$_{2}$OHCl-WMM and Li$_{2}$OHCl-WMM-210 ℃ sample show good thermal stability.

Fig. 3. (a) Typical Nyquist plots of Li$_{2}$OHCl-SSS (black), Li$_{2}$OHCl-WMM (blue), and Li$_{2}$OHCl-WMM-210 ℃ (red). (b) Arrhenius plots of temperature versus ionic conductivity for Li$_{2}$OHCl-SSS, Li$_{2}$OHCl-WMM and Li$_{2}$OHCl-WMM-210 ℃.

Figure 3(a) shows the Nyquist plots of Li$_{2}$OHCl measured at 25 ℃. The diameter of semiarch is used to estimate the resistance of Li$_{2}$OHCl, and the total ionic conductivity is calculated based on the pellet thickness and diameter. Arrhenius plot of ionic conductivity versus temperature for Li$_{2}$OHCl is given in Fig. 3(b). In addition, the activation energy $E_{\rm a}$ is obtained based on the equation $$ \sigma T=Ae^{(-E_{\rm a}/k_{\scriptscriptstyle{\rm B}}T)},~~ \tag {1} $$ where $A$ is the pre-exponential parameter, $\sigma$ is conductivity, $E_{\rm a}$ is activation energy, $k_{\scriptscriptstyle{\rm B}}$ and $T$ represent the Boltzmann constant and temperature, respectively. The lithium ionic conductivity of Li$_{2}$OHCl-SSS is $2.37 \times 10^{-7}$ S/cm at 25 ℃. The $E_{\rm a}$ for Li$_{2}$OHCl-SSS is 0.5231 eV between 50 ℃ and 120 ℃, which is consistent with the previous studies for Li$_{2}$OHCl.[10,15] It is noted that the $E_{\rm a}$ values for these samples are very close because they have the same cubic structure above 50 ℃. Below 50 ℃, for Li$_{2}$OHCl-WMM and Li$_{2}$OHCl-WMM-210 ℃, they could remain cubic phase in lower temperature compared to Li$_{2}$OHCl-SSS, which results in the ionic conductivity of the Li$_{2}$OHCl-SSS to be lower than the values of Li$_{2}$OHCl-WMM and Li$_{2}$OHCl-WMM-210 ℃. In view of structure, Li$^{+}$ could diffuse through the vacancies of the OH$^{-}$–Li octahedra. Hence, the cubic phase Li$_{2}$OHCl with high symmetry three-dimensional structure could boost ionic diffusion with low activation energy, while the orthorhombic-phase Li$_{2}$OHCl with highly anisotropic and two-dimensional structure restricts the transport of lithium ion, resulting in the low conductivity. In previously reports, the molecular dynamics simulation indicated that cubic phase provides larger space for OH$^{-}$ rotation to boost Li$^{+}$ hopping than orthorhombic phase. For Li$_{2}$OHCl-WMM, the lithium ionic conductivity increases to $4.27 \times 10^{-6}$ S/cm at 25 ℃ compared to Li$_{2}$OHCl-SSS ($2.37 \times 10^{-7}$ S/cm). The higher ionic conductivity is attributed to the cubic phase obtained by WWM at room temperature. More importantly, the sudden decrease of ionic conductivity occurs at 10 ℃ for Li$_{2}$OHCl-SSS, which is lower than the value of Li$_{2}$OHCl-SSS at 50 ℃. These experimental results illustrate that Li$_{2}$OHCl-WMM can work in a wider temperature range as solid electrolyte. Due to the fact that Li$_{2}$OHCl-WMM-210 ℃ still remains cubic phase after heating to 210 ℃, its ionic conductivity is higher than that of Li$_{2}$OHCl-SSS. Upon the temperature 30 ℃, the ion conductivity changes suddenly. On the other hand, at higher temperature, these three samples remain cubic phase, which cause very close $E_{\rm a}$ and ionic conductivity for them. In this study, both ac impedance and PXRD measurements reveal that cubic phase Li$_{2}$OHCl could be stabilized at room temperature by the wet mechanical milling method, although the Li$_{2}$OHCl usually forms orthorhombic phase at room temperature. The cubic phase Li$_{2}$OHCl shows higher ionic conductivity than orthorhombic phase at room temperature. This is mainly due to fact that the mechanical ball milling method can effectively synthesize the metastable structure. It has been reported that mechanical milling method could be used as an effective method to synthesize metastable structures, such as metal alloys,[16] metal hydrides,[17] and metal oxides.[18] During mechanical milling, the materials suffer from large stress that lead to generally lower crystallinity of materials than those materials synthesized by a sintering process at high temperature, therefore the low crystallinity would contribute to stabilized metastable phase at room temperature. As shown in Fig. 4, among those samples, Li$_{2}$OHCl-SSS shows the highest crystallinity, the largest crystal grains and the smoothest surface. On the contrary, Li$_{2}$OHCl-WMM shows the worst crystallinity, the smallest crystal grains and the roughest crystal surface. Crystallinity, grain size, and surface roughness of Li$_{2}$OHCl-WMM-210 ℃ are at a medium level. Li$_{2}$OHCl-WMM with the smallest grain size has the largest surface energy. On the other hand, from the viewpoint of atomic structure, the OH of the orthorhombic Li$_{2}$OHCl at room temperature is arranged in an orderly orientation, which leads to the lattice constants $a = b < c$. As the temperature increases, H has more kinetic energy and leads to a disordered arrangement of OH orientation, which leads to the lattice constant $a = b = c$, transforming from the orthorhombic phase to the cubic phase. We think that the larger surface energy of Li$_{2}$OHCl-WMM leading to reduction of the OH orientation order and the changing of crystal lattice inhibits transformation from the cubic phase to the orthorhombic at room temperature. However, as the temperature further decreases, the order orientation of OH becomes stronger. When the surface energy is not enough to suppress the lattice change, the transformation from cubic phase to orthorhombic phase would occurs. Therefore, the wet mechanical milling Li$_{2}$OHCl compared with the solid-state sintered Li$_{2}$OHCl shows a lower phase-transition temperature.

Fig. 4. SEM images for the Li$_{2}$OHCl synthesized by (a) solid-state sintering, (b) wet mechanical milling and 210 ℃ heating, and (c) wet mechanical milling.

In summary, we have proposed a novel wet mechanical milling method to synthesize cubic phase Li$_{2}$OHCl, of which the ionic conductivity is one order of magnitude higher than that of thermodynamically stable orthorhombic phase at room temperature. Moreover, the phase-transition temperature decreases from 40 ℃ to 10 ℃ by the wet mechanical milling method. This means that the Li$_{2}$OHCl can perform higher ionic conductivity at room temperature and work stably in a wider temperature range as solid electrolyte. Furthermore, the wet mechanical milling method with simple preparation process shows a huge potential to fabricate anti-perovskite solid-state electrolyte. Acknowledgements. This work was supported by the Guangdong Basic and Applied Basic Research Foundation (Grant No. 2021A1515011784), the Key Program of the National Natural Science Foundation of China (Grant No. 51732005), the Shenzhen Science and Technology Program (Grant No. KQTD20200820113047086), 21C Innovation Laboratory, Contemporary Amperex Technology Ltd (Grant No. C-ND-21C LAB-210044-1.0), and the Guangdong-Hong Kong-Macao Joint Laboratory for Neutron Scattering Science and Technology. References Self‐Suppression of Lithium Dendrite in All‐Solid‐State Lithium Metal Batteries with Poly(vinylidene difluoride)‐Based Solid ElectrolytesRecent advances in all-solid-state rechargeable lithium batteriesA complex hydride lithium superionic conductor for high-energy-density all-solid-state lithium metal batteriesEffects of preparation conditions on the ionic conductivity of hydrothermally synthesized Li1+Al Ti2-(PO4)3 solid electrolytesRetraction: “Lithium-ion conducting La _{2/3– x} Li _{3 x} TiO ₃ solid electrolyte thin films with stepped and terraced surfaces” [Appl. Phys. Lett. 100, 173107 (2012)]Electrochemical Stability of Li ₁₀ GeP ₂ S ₁₂ and Li ₇ La ₃ Zr ₂ O ₁₂ Solid ElectrolytesLi ₂ OHCl Crystalline Electrolyte for Stable Metallic Lithium AnodesFluorine‐Doped Antiperovskite Electrolyte for All‐Solid‐State Lithium‐Ion BatteriesLocal Structural Changes and Inductive Effects on Ion Conduction in Antiperovskite Solid ElectrolytesCrystal structures and ionic conductivity in Li2OHX (X = Cl, Br) antiperovskitesFundamental aspects of the structural and electrolyte properties of

{Li}_{2} OHCl

from simulations and experimentProtons Enhance Conductivities in Lithium Halide Hydroxide/Lithium Oxyhalide Solid Electrolytes by Forming Rotating Hydroxy GroupsUnderstanding Li‐Ion Dynamics in Lithium Hydroxychloride (Li ₂ OHCl) Solid State Electrolyte via Addressing the Role of ProtonsHigh Lithium Ionic Conductivity in the Lithium Halide Hydrates Li3-n(OHn)Cl (0.83≤n≤2) and Li3-n(OHn)Br (1≤n≤2) at Ambient TemperaturesSynthesis of the Metastable Cubic Phase of Li ₂ OHCl by a Mechanochemical MethodMetastable phase transformations induced by ball-milling in the CuW systemDiscovering a new MgH ₂ metastable phaseFormation of metastable phases by high-energy ball milling in the Ti-O system

[1]	Zhang X, Wang S, Xue C, Xin C, Lin Y, Shen Y, Li L, and Nan C W 2019 Adv. Mater. 31 1806082
[2]	Sun C, Liu J, Gong Y, Wilkinson D P, and Zhang J 2017 Nano Energy 33 363
[3]	Kim S, Oguchi H, Toyama N et al. 2019 Nat. Commun. 10 1081
[4]	Kim K M, Shin D O, and Lee Y G 2015 Electrochim. Acta 176 1364
[5]	Ohta H, Mizoguchi T, Aoki N, Yamamoto T, Sabarudin A, and Umemura T 2013 Appl. Phys. Lett. 102 089902
[6]	Han F, Zhu Y, He X, Mo Y, and Wang C 2016 Adv. Energy Mater. 6 1501590
[7]	Hood Z D, Wang H, Samuthira P A, Keum J K, and Liang C 2016 J. Am. Chem. Soc. 138 1768
[8]	Li Y, Zhou W, Xin S, Li S, Zhu J, Lu X, Cui Z, Jia Q, Zhou J, Zhao Y, and Goodenough J B 2016 Angew. Chem. Int. Ed. Engl. 55 9965
[9]	Deng Z, Ou M, Wan J et al. 2020 Chem. Mater. 32 8827
[10]	Koedtruad A, Patino M A, Ichikawa N, Kan D, and Shimakawa Y 2020 J. Solid State Chem. 286 121263
[11]	Howard J, Hood Z D, and Holzwarth N A W 2017 Phys. Rev. Mater. 1 75406
[12]	Song A Y, Xiao Y, Turcheniuk K, Upadhya P, Ramanujapuram A, Benson J, Magasinski A, Olguin M, Meda L, Borodin O, and Yushin G 2018 Adv. Energy Mater. 8 1700971
[13]	Song A Y, Turcheniuk K, Leisen J, Xiao Y, Meda L, Borodin O, and Yushin G 2020 Adv. Energy Mater. 10 1903480
[14]	Georg S M J 2003 ChemPhysChem 4 343
[15]	Yamamoto T, Shiba H, Mitsukuchi N, Sugumar M K, Motoyama M, and Iriyama Y 2020 Inorg. Chem. 59 11901
[16]	Gaffet E, Louison C, Harmelin M, and Faudot F 1991 Mater. Sci. Eng. A 34 1380
[17]	El-Eskandarany M S, Banyan M, and Al-Ajmi F 2018 RSC Adv. 8 32003
[18]	Hara K O, Yamasue E, Okumura H, and Ishihara K N 2009 J. Phys.: Conf. Ser. 144 012021