the following active materials were employed in this work : li4ti5o12 ( d50 1.9 m ) from mti corporation , licoo2 ( 710 m ) from sigma  aldrich , and anatase tio2 ( < 10 nm ) from sachtleben chemie .
all components of the electrolyte , namely ethylene carbonate and dimethyl carbonate and lipf6 , and metallic lithium were purchased from sigma  aldrich ( battery grade ) .
the oxygen - annealed li4ti5o12 samples ( o_lto ) were obtained by treatment commercially available li4ti5o12 ( p_lto ) in flowing o2 ( 100 vol % ) at 650 c for 120 min . the o2-annealed sample ( o_lto )
was then treated in flowing h2 ( 5 vol % in ar ) at 650 c for 90 min to obtain hydrogen - annealed li4ti5o12 ( h_lto ) .
the bet surface areas were determined by nitrogen physisorption using a tristar ii 3020 from micromeritics .
high resolution transmission electron microscopy ( hrtem ) analyses were carried out with a field emission gun microscope fei tecnai f20 , working at 200 kv and with a point - to - point resolution of 0.19 nm .
electron energy loss spectroscopy ( eels ) spectra were obtained in a gatan quantum detector coupled to the f20 microscope .
x - ray power diffraction ( xrd ) analyses were carried out on a bruker axs d8 advance x - ray diffractometer with cuk1 radiation ( =1.5406  ) .
the uv / vis spectra were recorded in diffuse reflectance mode in a perkinelmer lambda 950 uv / vis spectrometer equipped with a 150 mm int .
the chemical composition of the surface of the lto was analyzed by x - ray photoelectron spectroscopy ( xps ) using a phi instrument model 5773 multitechnique with a1k radiation ( 1486.6 ev ) .
the fluid electrodes were prepared by mixing the active materials and carbon additive in the electrolyte ( ec : dmc in 1 m lipf6 ) by magnetic stirring for overnight .
the fluid electrode was flowed through the cell for , at least , 1 h before starting the experiments to further homogenize the suspension .
for the suspensions , 2.5 g and 0.125 g of active and conductive additive , respectively , were added into 12 ml for tio2 and licoo2 and into 6 ml for lto .
the poor electrical conductivity and low surface area of lto forced us to increase the concentration of particles in the suspension to obtain decent performances .
for solid electrodes , a slurry containing 75:15:10 wt % of active material , conductive additive and poly(vinylidene difluoride ) ( pvdf ) binder ( sigma  aldrich ) was prepared by dispersing in n - methyl pyrrolidone ( nmp ) ( sigma  aldrich ) and mixing for overnight using magnetic stirring .
the slurry was doctor - bladed onto copper foil ( schlenk ) and dried at 60 c for 10 h in air , resulting in an active material loading of about 3 mg cm .
10 mm disk electrodes were punched out using a commercially available hole punch ( hoffmann ) and dried overnight at 80 c in a vacuum oven the electrochemical cells for the characterization of both solid and fluid electrode were two - electrode configuration and were assembled in an ar - filled glove box ( o2<1 ppm and h2o<1 ppm ) .
celgard 2500 and ec : dmc in 1 m lipf6 served as separators and electrolyte , respectively . the electrochemical cell employed for testing fluid electrodes
the dimensions of the channel were 7541 mm and the current collector was metallic copper .
electrochemical measurements were performed with a bio - logic vmp-3 ( bio logic sas ) .
films containing 90 % of lto and 10 % of poly(vinylidene difluoride ) ( pvdf ; binder ) were prepared on copper foil by the doctor blade technique .
45 micrometer , which was measured with a digital external micrometer ( holex 421505 ) .
discs of 12 mm diameter were cut and the electrical conductivity was estimated by 2 probe measurement using a multimeter ( uni - t ut70a ) .
the measurements for electrical conductivity were carried out using the same cell employed for the electrochemical evaluation of fluid electrode , with the exception of the separators and the metallic lithium .
the electrical resistivity of the suspension was then estimated with multimeter ( silver electronics ut33d ) knowing the section and the length of the cell ( 3 cm1 mm ) .
note that the objective of these measurements was to compare the electrical conductivities of the three suspension ( p_lto , o_lto and h_lto ) , rather than obtaining the exact values . to avoid changing the cell configuration factor for the different samples
, the cell was not disassembled between the three experiments but it was vigorously cleaned with pure dmc .
the solid electrode prepared for the measurement of the electrical conductivity of the active material ( 90:10 lto / binder ) were also employed for eis . a typical three - electrode swagelok cell using metallic lithium as counter and reference electrode was used . whatman and ec /
the cell was assembled in an argon - filled glove - box , then taken outside the glove - box and tested with a bio - logic vmp-3 ( bio logic sas ) .
the state of charge was tuned galvanostatically at 0.1 c. the eis were performed in a frequency range of 100 khz0.020 hz with an amplitude of 20 mv .
the following active materials were employed in this work : li4ti5o12 ( d50 1.9 m ) from mti corporation , licoo2 ( 710 m ) from sigma  aldrich , and anatase tio2 ( < 10 nm ) from sachtleben chemie .
all components of the electrolyte , namely ethylene carbonate and dimethyl carbonate and lipf6 , and metallic lithium were purchased from sigma  aldrich ( battery grade ) .
the oxygen - annealed li4ti5o12 samples ( o_lto ) were obtained by treatment commercially available li4ti5o12 ( p_lto ) in flowing o2 ( 100 vol % ) at 650 c for 120 min . the o2-annealed sample ( o_lto )
was then treated in flowing h2 ( 5 vol % in ar ) at 650 c for 90 min to obtain hydrogen - annealed li4ti5o12 ( h_lto ) .
the bet surface areas were determined by nitrogen physisorption using a tristar ii 3020 from micromeritics .
high resolution transmission electron microscopy ( hrtem ) analyses were carried out with a field emission gun microscope fei tecnai f20 , working at 200 kv and with a point - to - point resolution of 0.19 nm .
electron energy loss spectroscopy ( eels ) spectra were obtained in a gatan quantum detector coupled to the f20 microscope .
x - ray power diffraction ( xrd ) analyses were carried out on a bruker axs d8 advance x - ray diffractometer with cuk1 radiation ( =1.5406  ) .
the uv / vis spectra were recorded in diffuse reflectance mode in a perkinelmer lambda 950 uv / vis spectrometer equipped with a 150 mm int .
the chemical composition of the surface of the lto was analyzed by x - ray photoelectron spectroscopy ( xps ) using a phi instrument model 5773 multitechnique with a1k radiation ( 1486.6 ev ) .
the fluid electrodes were prepared by mixing the active materials and carbon additive in the electrolyte ( ec : dmc in 1 m lipf6 ) by magnetic stirring for overnight .
the fluid electrode was flowed through the cell for , at least , 1 h before starting the experiments to further homogenize the suspension .
the reservoirs were constantly stirred during the electrochemical characterization . for the suspensions , 2.5 g and 0.125 g of active and conductive additive , respectively , were added into 12 ml for tio2 and licoo2 and into 6 ml for lto .
the poor electrical conductivity and low surface area of lto forced us to increase the concentration of particles in the suspension to obtain decent performances .
for solid electrodes , a slurry containing 75:15:10 wt % of active material , conductive additive and poly(vinylidene difluoride ) ( pvdf ) binder ( sigma  aldrich ) was prepared by dispersing in n - methyl pyrrolidone ( nmp ) ( sigma  aldrich ) and mixing for overnight using magnetic stirring .
the slurry was doctor - bladed onto copper foil ( schlenk ) and dried at 60 c for 10 h in air , resulting in an active material loading of about 3 mg cm .
10 mm disk electrodes were punched out using a commercially available hole punch ( hoffmann ) and dried overnight at 80 c in a vacuum oven
the electrochemical cells for the characterization of both solid and fluid electrode were two - electrode configuration and were assembled in an ar - filled glove box ( o2<1 ppm and h2o<1 ppm ) .
celgard 2500 and ec : dmc in 1 m lipf6 served as separators and electrolyte , respectively . the electrochemical cell employed for testing fluid electrodes
the dimensions of the channel were 7541 mm and the current collector was metallic copper .
electrochemical measurements were performed with a bio - logic vmp-3 ( bio logic sas ) .
films containing 90 % of lto and 10 % of poly(vinylidene difluoride ) ( pvdf ; binder ) were prepared on copper foil by the doctor blade technique .
45 micrometer , which was measured with a digital external micrometer ( holex 421505 ) .
discs of 12 mm diameter were cut and the electrical conductivity was estimated by 2 probe measurement using a multimeter ( uni - t ut70a ) .
the measurements for electrical conductivity were carried out using the same cell employed for the electrochemical evaluation of fluid electrode , with the exception of the separators and the metallic lithium .
the electrical resistivity of the suspension was then estimated with multimeter ( silver electronics ut33d ) knowing the section and the length of the cell ( 3 cm1 mm ) .
note that the objective of these measurements was to compare the electrical conductivities of the three suspension ( p_lto , o_lto and h_lto ) , rather than obtaining the exact values . to avoid changing the cell configuration factor for the different samples
, the cell was not disassembled between the three experiments but it was vigorously cleaned with pure dmc .
the solid electrode prepared for the measurement of the electrical conductivity of the active material ( 90:10 lto / binder ) were also employed for eis . a typical three - electrode swagelok cell using metallic lithium as counter and reference electrode was used . whatman and ec /
the cell was assembled in an argon - filled glove - box , then taken outside the glove - box and tested with a bio - logic vmp-3 ( bio logic sas ) .
the state of charge was tuned galvanostatically at 0.1 c. the eis were performed in a frequency range of 100 khz0.020 hz with an amplitude of 20 mv .
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 the semi - solid flow battery ( ssfb ) is a promising storage energy technology featured by employing semi - solid fluid electrodes containing conductive additive and active li - ion battery materials . 
 the state of art anode material for ssfb is li4ti5o12 ( lto ) . 
 this work shows that lto improves drastically the performance in fluid electrode via hydrogen annealing manifesting the importance of the electrical conductivity of the active material in ssfbs . on the other hand , 
 the properties of fluid electrodes allow the contributions of ionic and electrical resistance to be separated in operando . 
 the asymmetric overpotential observed in li4ti5o12 and tio2 is proposed to originate from the so - called electron bottleneck mechanism based on the transformation from electrically insulator to conductor upon ( de-)lithiation , or vice versa , which should be considered when modelling , evaluating or designing advanced materials based on li4ti5o12 , tio2 or others with insulating - conducting behavior materials . 
Experimental Section
Materials and thermal treatment
Structural characterization
Suspension preparation
Electrochemical characterization
Electrical conductivity of active material
Electrical conductivity test of suspensions
Electrochemical impedance spectroscopy (EIS)
Supporting Information