21 2 Lithium-ion batteries (LIB) are integrated in a wide range of electronic devices that are an 22 integral part of our modern world. Growing number of LIBs that reach their end of life 23 demands development of effective recycling strategies to recover rare and/or expensive 24 battery materials. Dielectrophoresis (DEP) is an electrokinetic particle manipulation 25 technique that allows for selective particle separation based on properties, such as material, 26 size, and shape. Here, we demonstrate separation of lithium iron phosphate (LFP) and 27 graphite using dielectrophoretic filtration. Graphite and LFP are two common LIB anode and 28 cathode materials. We demonstrate both: non-selective separation using pure suspensions 29 of both graphite and uncoated LFP and an isolation of graphite from a mixture of uncoated 30 LFP and graphite. We confirmed that LFP shows negative DEP while graphite shows positive 31 DEP. We determined the conductivity at which material-selective polarisability-based 32 separation becomes possible, thus, proofing the concept of sorting of non-carbon coated 33 lithium iron phosphate (LiFePO!) and graphite. These results reinforce one possibility of using 34 DEP filtration as a potential method for direct physical recycling of battery material waste. 35 1. Introduction 36 Lithium-ion batteries (LIBs) are used in many electronic devices that surround our everyday 37 life. The constantly growing number of LIBs that reach their end of life demands development 38 of an appropriate recycling procedure. Cathode active materials are usually lithium metal 39 oxides while the anode active material is graphite. Demand of graphite in the context of LIB 40 production is 10-20 times higher than that of lithium [1]. To reduce dependence on natural or 41 synthetic virgin graphite, proper recycling methods for this material are essential [1]. The 42 most common methods for LIB recycling are pyrometallurgy, hydrometallurgy, and direct 43 physical [2]. Pyrometallurgy does not require any pre-treatment but suffers from large energy 44 3 needs and can cause environmental pollution with toxic gases that are produced during the 45 process. Additionally, for a full recovery, pyrometallurgy must always be followed by a 46 hydrometallurgical step to treat the alloy [3]. Hydrometallurgy involves huge consumption of 47 chemical reagents [2]. Both hydrometallurgy and pyrometallurgy focus only on the recovery 48 of the main cathode components, graphite is lost in both processes. Direct physical recycling 49 processes focus on using particle technology to directly separate and recover the cathode 50 and anode active material, however, universally applicable methods to achieve this task are 51 lacking. 52 Here, we research dielectrophoresis (DEP) as a method to separate cathode and anode 53 material. DEP allows for a particle separation based on polarisability. Our group developed 54 several approaches to treat particle systems at high throughput using DEP [4–6]. Here, we 55 are expanding the scope of dielectrophoretic filtration towards polarisability-dependent 56 separation of LIB materials of different conductivities. Dielectrophoretic filtration [5,7-11] is a 57 macroscopic high-throughput DEP method based on applying an electric field across a 58 porous dielectric material (filter). This filter disturbs the electric field, creating local field 59 extrema which can be used to selectively trap particles by dielectrophoresis. The 60 dielectrophoretic force is proportional to the electric field strength and linearly dependent on 61 the Clausius-Mossotti (CM) factor [12]. The CM factor is influenced by the complex 62 permittivities of the particle and the medium in which the particle is submerged. At low 63 frequencies, the CM factor depends only on the medium and particle conductivities. 64 Therefore, DEP can be used for material-selective separation by controlling and tuning the 65 conductivity of the suspension medium. When medium conductivity becomes significantly 66 higher than the conductivity of the target particles, the particles are getting pushed away 67 from electric field maxima, they are said to experience negative dielectrophoresis (nDEP). 68 4 When the medium conductivity is lower than the particle conductivity, then the particles are 69 attracted towards the local field maxima, they are said to experience positive DEP (pDEP). 70 Particles that experience pDEP will become immobilized in local field maxima in the filter and 71 we can use DEP filtration to selectively trap target particles. It is possible to recover the 72 trapped particles by switching off the electric field. In another work from our group, graphite 73 particles were isolated from significantly smaller carbon black-coated LFP particles (i.e., 74 conductive particles) through pDEP using electrode-based dielectrophoresis [6]. While both 75 particles showed pDEP, separation was possible because the DEP force is volume 76 dependent. Here, we show that graphite particles can be isolated from uncoated, i.e., non-77 conductive, LFP particles of comparable size using dielectrophoretic filtration. Thus, we show 78 polarisability-dependent sorting of particles. We show that we can separate graphite powder 79 from a mixture of graphite and uncoated lithium-iron phosphate powder in aqueous 80 suspension. While this study aims to expand the scope of DEP filtration, we also pave the 81 towards solving an important problem in the recycling of LIB, i.e., the selective recovery of 82 graphite from LIB waste. 83 2. Material and Methods 84 The main components of our setup are the particle suspension, filter cell and analysis module 85 (Fig. 1). 86 5 87 Figure 1. DEP filtration, a schematic setup of the filtration experiment. 88 The core of the setup, the DEP filter unit, is identical to the one used in Ref. [7, 9]. The filter 89 cell has dimensions of 8×29×18 mm" and is made of polytetrafluoroethylene. Here, we are 90 using a packed bed filter of grained silica material (sand) as filter matrix. The matrix is 91 characterized in detail in Ref. [7]. The pores of the filter are ≥20 times larger than the size 92 of the investigated particles. Two macroscopic electrodes, made of the stainless steel, are 93 mounted inside the filter cell region at a distance of 8 mm. An ac electric field was applied 94 across these electrodes. The signal was generated by a function generator (HM8131, 95 Hameg Instruments GmbH, Germany) and amplified by a voltage amplifier (PZD700A, TREK 96 Inc., USA). The resulted voltage on the setup was measured using the power analyser 97 LMG670 (ZES ZIMMER ElectronicSystems GmbH, Germany). The suspension was 98 delivered to the filter cell with a peristaltic pump (REGLO Analog,Ismatec, Switzerland). 99 6 During all the experiments, the flow rate was constant and kept at 360 mL/h. The connecting 100 tubes are accompanied with a three-way valve, allowing for the suspension to go either via 101 the filter cell to the analysis module, or via the bypass to the analysis module. 102 103 Figure 2. Particle size distribution (PSD), number-based and additionally normalized 104 distribution of two particle systems (LFP and graphite), measured three times each with laser 105 diffraction. 106 Particle suspensions were uncoated lithium iron phosphate (LFP) and graphite. Virgin 107 material was purchased commercially in the form of the fine powders with particle size less 108 than 5 µm (company information) from Sigma-Aldrich. Graphite (TIMCAL KS-6) was 109 purchased as powder with mean particle size ≤3.4 µm (company information) from MSE 110 Supplies. Before experimentation, we additionally measured the particle size distributions 111 using laser diffraction (LD, Mastersizer 2000, Malvern Panalytical GmbH, United Kingdom), 112 see Fig. 2 (more details are in supplementary materials), and found that the results fall close 113 into the size range given by the companies. Pure particle suspensions were prepared by 114 suspending particle in pure deionized water (Omniatap 6 UV/UF, Starkpure GmbH, 115 Germany) to a concentration 4 mg/L (graphite) and 8 mg/L (LFP). We further added Tween 116 7 20 solution (Sigma-Aldrich, Germany) to a final concentration of 0.004% vol. The conductivity 117 of the final suspension was tuned by adding 0.1 M KCl to a final value of 1–15 µS/cm. 118 Through the experimental stage, the suspension was always disturbed with a stirring 119 magnet. For the preparation of the mixture suspension, graphite and LFP were mixed into 120 one beaker in the same concentrations as they were in separate suspensions before. 121 The trapping efficiency of the separate mixtures was qualitatively measured using a 122 spectrometer (HORIBA, FluorMAX-4) on-line reflection measurement combined with 123 MATLAB processing [7, 13]. The suspension was flowing through a quartz cuvette (176.762-124 QS, Hellma). A qualitative estimate of (non-selective) separation efficiency is given by on-125 line measuring the reflection of the particles at the outlet of the setup. The particles are not 126 ideal spheres (supplier information) so that we can only qualitatively measure their 127 concentration like this. The processing of the signal was done similar to our previous work, 128 by the use of the MATLAB script from the previous study, which calculates the trapping 129 efficiency by subtracting from 100 % a ratio of the signal from the particles going freely to the 130 filter, to the signal of the particles, recorded while the electric field was on [7, 13]. 131 To address quantitatively the separation efficiency of LFP in a mixture with graphite, we used 132 atomic absorption spectroscopy (AAS). For AAS, we collected a defined volume of around 133 20 mL of the suspension into 50 mL volumetric flasks during the different stages of the 134 experiment. The LFP was dissolved by adding 3 mL 65 % nitic acid (VWR International, 135 Belgium) and 3 mL 30 % hydrogen peroxide (Sigma-Aldrich, Germany). After the volumetric 136 flasks were topped up with pure deionized water, the lithium content inside the prepared 137 mixture was determine with AAS using a Solaar 989 QZ AA Spectrometer (Unicam, England) 138 with a GF90+ furnace and an FS90+ autosampler. 139 8 3. Results and Discussion 140 The graphite particles have a significantly higher bulk conductivity value than the aqueous 141 suspension and we expect them to show pDEP behaviour. The uncoated LFP particles, 142 however, have a low conductivity value of around 10-7–10-8 S/cm, which is lower than that of 143 the medium and they should therefore demonstrate nDEP behaviour [14, 15]. 144 We have firstly investigated if LFP indeed shows negative DEP in a simple setup. A 50 µL 145 drop of the LFP suspension at 30 µS/cm conductivity was placed on top of an interdigitated 146 electrode array and we directly observed particle movement using a microscope. For this, a 147 voltage of 20 V#$% at 15 kHz was applied with a signal generator (Rigol DG4062, Rigol 148 Technologies EU GmbH, Puchheim, Germany). The particle movement was recorded using 149 an inverted microscope (ECLIPSE Ts2R-FL, Nikon Instruments Europe BV, Amsterdam, 150 Netherlands) and a CMOS camera (Grasshopper GS3-U3-51S5C-C, FLIR Systems Inc., 151 Wilsonville, OR, USA). The interdigitated electrodes have an arm width and gap width of 100 152 μm and were fabricated using standard cleanroom techniques. Upon application of the 153 voltage, LFP particles indeed accumulated in the space between the electrodes, which is 154 typical behaviour for particles that show nDEP (see Fig. 3 and the video in the supplement). 155 156 Figure 3. Behaviour of LFP (black particles) in an array of interdigitated electrodes at 20 V 157 and 15 kHz and at a conductivity of 30 µS/cm. Upon application of an electric field, the 158 9 randomly distributed LFP particles (left) are predominantly repelled to the space between the 159 electrodes (right). This is typical behaviour for particles that experience nDEP. Electrode 160 distance is 100 µm. 161 We then performed LFP and graphite separation experiments with our filter cell setup. In 162 individual (non-selective) experiments, at 5 µS/cm, the trapping of both graphite increases 163 with voltage, as expected from the DEP theory for pDEP particles (Fig. 4, left). The trapping 164 of LFP also increases with voltage, which might be due to to nDEP retention, as we have 165 previously observed [9]. 166 167 Figure 4. DEP trapping efficiency of two separate powders, analysed with the reflection 168 measurement and an additional result of trapping of only LFP from the mixture (*) at 15 kHz, 169 360 mL/h flow rate. Left: Dependence on the voltage (at 5 𝜇𝑆/𝑐𝑚 conductivity); right: 170 dependence on the conductivity (at 350 𝑉'() voltage) with their respective standard 171 deviations from three repetitions. 172 While at 300 Vrms, LFP shows almost no trapping, it does show appreciable separation of 173 almost 20 % at 500 Vrms. Further, we observed that LFP trapping decreases with increasing 174 10 solution conductivity (Fig. 4, right). While at 1 µS/cm and 350 Vrms, LFP shows trapping above 175 30 %, it drops to 1% at 15 µS/cm. Graphite trapping efficiency also drops slightly with 176 increasing conductivity, from 90% at 1 µS/cm to 70% at 15 µS/cm. Thus, at 15 µS/cm, LFP 177 trapping is negligible while graphite trapping is high. The mechanism why LFP shows higher 178 trapping at lower conductivities is unclear. Its bulk conductivity value should be well below 1 179 µS/cm and from microscopy experiments, we know that LFP shows nDEP at 30 µS/cm. We 180 could not perform microscopy experiments at lower conductivities, as the required particle 181 load was too high to achieve lower conductivities. The small decline of the trapping efficiency 182 for graphite is believed to be associated with thermal effects at the higher conductivities of 183 aqueous suspension. Such an explanation was given for a similar observation with 184 polystyrene particles, assuming that an increase in conductivity raises temperature due to 185 more energy dissipation and this in turn increases a natural convection of the fluid creating 186 more “undirected” fluid motion [9]. 187 To separate graphite from a mixture of graphite and LFP of similar size, both particles were 188 added in one suspension. The trapping efficiency of LFP was evaluated at 15 kHz, 350 𝑉*+, 189 using the AAS protocol at two solution conductivities, 1 and 15 µS/cm, respectively. The 190 trapping efficiency of LFP in these experiments is plotted as stars in Fig. 4. Similar to the 191 pure particle experiments, LFP showed a trapping efficiency of roughly 1 % at 15 µS/cm and 192 of 15 % at 1 µS/cm. The difference between trapping efficiency in pure particle and mixture 193 experiments (i.e., black dots vs. black stars in the right Fig. 4) is probably because two 194 different measurement techniques were used. As mentioned, reflection measurement can 195 only give a qualitative estimate of separation, while AAS actually measures lithium content 196 and gives more precise results, as also observed before [6]. We did not measure the graphite 197 11 concentration in mixture experiments and thus assume that graphite shows a separation 198 efficiency similar to pure particle experiments. 199 4. Conclusions 200 Our research indicates the potential of dielectrophoretic filtration as an additional purification 201 step for recycling of battery materials. Our findings demonstrate that by manipulation of the 202 conductivity of the liquid medium, we can selectively separate graphite from a mixture of 203 graphite and uncoated LFP using DEP filtration. This is based on differences in the 204 polarisability of both particles. Additional research is required to further estimate the 205 technique in use. Especially, it is essential to evaluate the proper recovery rate of the target 206 particles and to tune the filter matrix geometrical parameters to avoid unwanted nDEP 207 trapping of non-target particles. It's important to note that while we successfully separated 208 commercially available particles, the application to real black mass with graphite-coated LFP 209 poses additional challenges. 210 Acknowledgements 211 The authors would like to thank Jasper Giesler, Laura Weirauch, Swantje Drolshagen 212 (University of Bremen) and Askar Kvaratsheliya (Leibniz Institute for Materials Engineering, 213 IWT) for their assistance. 214 215 Author contributions 216 Mariia Kepper: conceptualization, project administration, methodology, investigation, 217 validation, visualization, formal analysis, writing – original draft, review & editing. Alica 218 12 Rother: investigation (AAS), data curation, formal analysis (AAS). Jorg Thöming: 219 methodology, resources, review & editing. Georg Pesch: conceptualization, project 220 administration, methodology, validation, supervision, writing - review & editing. 221 Data Availability 222 An online repository (referred as supplementary materials in this text) is accompanying this 223 article and contains of details on the evaluation of DEP data, additional details on the 224 measurement of the particle-size distribution data of the LFP and graphite, notes on AAS 225 analysis and a video demonstrating nDEP of LFP [13]. Additional information is available 226 from the corresponding author upon a reasonable request. 227 Conflict of interest 228 The authors declare no competing interests. 229 5.