2. Single-ion conducting soft electrolytes for all-solid-state lithium metal batteries operating under
2.3. Results and discussion
2.3.4. Beyond conventional ASSLMBs
To develop a high-energy-density S-ASSLMB full cell, the above-prepared NCM811 cathode was coupled with a thin Li metal anode (~20 µm) in the presence of SSE (Figure 2.22a). In addition, the charge cut-off voltage was raised to 4.6 V. Under this harsh cut-off voltage, the obtained S-ASSLMB full cell showed decent charge/discharge cycling retention at ambient operating conditions (Figure 2.22b). The result of the S-ASSLMB was compared with those of previously reported ASSLMB full cells (Table 2.3). The previous works have often used thick Li metal anodes and thick solid-state electrolytes to ensure electrochemical reliability of the ASSLMB full cells, resulting in the loss of volumetric cell energy densities.
Single-ion conductors can improve low-temperature cell performance owing to a suppression of concentration polarization at the electrodes68. The S-ASSLMB showed stable charge/discharge profiles over a wide range of temperatures (−10 ~ 10°C) (Figure 2.23a), whereas the control ASSLMB failed to maintain electrochemical activity below −10°C. Notably, the S-ASSLMB successfully operated a light-emitting diode (LED) lamp under harsh temperature of −27.1°C (Figure 2.23b). This excellent low-temperature cell performance was verified by analyzing the EIS profiles of the cells at
−30°C (Figure 2.23c). The S-ASSLMB exhibited a lower SEI resistance (RSEI) and charge transfer resistance (Rct) than the control ASSLMB. Meanwhile, both the SSE (for S-ASSLMB) and C1 (for control ASSLMB) did not show any crystalline phases in differential scanning calorimetry thermograms (Figure 2.23d), revealing that the low-temperature cell performance of the S-ASSLMB is ascribed mainly to the single-ion conduction characteristic (tLi+ = 0.91) of the SSE and the stabilized SSE- electrode interfaces, not the differences in the bulk thermodynamic states between the SSE and C1.
One of the formidable challenges facing traditional ASSLMBs based on ISEs is the lack of mechanical flexibility. In sharp contrast, the S-ASSLMB, due to the highly deformable printed SSE, printed NCM811 cathode (Figure 2.24a and 2.24b), and monolithic integration of the cell components enabled by the multistage printing, is expected to be physically flexible. To investigate mechanical flexibility of the S-ASSLMB, a pouch-type cell (length × width = 18 × 18 (mm/mm)) was fabricated.
The obtained S-ASSLMB, after the 100th bending cycle (bending radius = 5 mm and deformation rate
= 30 mm min−1), showed no significant change in the charge/discharge profiles (Figure 2.24c).
Moreover, the S-ASSLMB powered an LED lamp even after being folded multiple times (inset of Figure 2.24d).
Despite the use of solid-state electrolytes in ASSLMBs, there have been very few studies reporting the cell safety, to the best of our knowledge. To address this issue, we examined the safety behavior of the S-ASSLMB. The fully charged S-ASSLMB was placed in a hot box (set to 100°C) and its voltage change was traced as a function of elapsed time. The S-ASSLMB maintained its voltage,
48
whereas a control cell with conventional liquid electrolyte (1 M LiTFSI in EC/DMC = 1/1 (v/v)) was severely swollen and dimensionally distorted (Figure 2.25a). After being horizontally cut in-half, the S-ASSLMB operated a LED lamp without explosion and structural disruption (Figure 2.25b). Notably, the cut S-ASSLMB continuously powered the LED lamp, even upon exposure to flame. Such exceptional safety of the S-ASSLMB is attributed to the nonflammable SSE (Figure 2.25c and 2.25d) enabled by the coordinated electrolyte (4M LiFSI in PC/FEC)31. These results underscored the viability of the SSE as a promising alternative to ISEs.
We explored the potential feasibility of the ASSLMB in bipolar cell configurations39 that can provide higher cell voltages. On top of the S-ASSLMB unit cell, UV curing-assisted multistage printing was repeatedly performed to fabricate bipolar S-ASSLMBs with an in-series configuration (Figure 2.26a). The bipolar S-ASSLMBs showed normal and stable charge/discharge profiles (at a charge/discharge current density of 0.1 C/0.1 C) over the entire stacked cells examined herein (Figure 2.26b). The working voltages of the bipolar S-ASSLMBs were linearly proportional to the number of serially stacked cells, exhibiting facile control of cell voltages through the bipolar cell configuration.
More notably, a cell voltage of 16.8 V was achieved with the bipolar S-ASSLMB (4 cells stacked in- series, Figure 2.26c), which has never been reported in the previous studies of ASSLMBs. These results demonstrated that the bipolar S-ASSLMBs can be used as a practical power source with tunable operating voltages.
A comparison (Table 2.3) with previously reported ASSLMBs showed that the S-ASSLMB enabled improvements in the mechanical flexibility, nonflammability, and the number of bipolar- stacked cells in addition to the electrochemical superiority (including the operating conditions, cyclability, rate capability, and low-temperature performance) described above, underscoring the validity of SSE as a promising solid-state electrolyte candidate for practical ASSLMBs.
49
Figure 2.22. High-voltage performance of the S-ASSLMBs. (a) Cross-sectional SEM image of the 4.6 V-charged S-ASSLMB (Li (~ 20 µm) anode||NCM811 cathode). (b) Charge/discharge voltage profiles of the 4.6 V-charged S-ASSLMB (Li (~ 20 µm) anode||NCM811 cathode) at a charge/discharge current density of 0.18/0.18 mA cm−2.
50
Figure 2.23. Low-temperature performance of the S-ASSLMBs. (a) Charge/discharge voltage profiles of the S-ASSLMB as a function of operating temperature (−10 ~ 10°C). (b) Photograph showing the operation of a LED lamp powered by the S-ASSLMBs at −27.1°C. (c) EIS spectra of the control ASSLMB and S-ASSLMB at −30°C. (d) DSC thermograms of the C1 and SSE at a scan rate of 5°C min−1.
51
Figure 2.24. Mechanical flexibility of the S-ASSLMBs. (a) Photographs showing the bending (bending radius = 5 mm and deformation rate = 30 mm min−1), winding, and multiple folding of the SSE. (b) Electronic resistance of the printed NCM811 cathode as a function of bending cycle (bending radius = 5 mm and deformation rate = 30 mm min−1). (c) charge/discharge voltage profiles of the S- ASSLMB before and after bending (bending radius = 5 mm and deformation rate = 30 mm min−1). (d) The multiple-folded S-ASSLMB powered an LED lamp.
52
Figure 2.25. Nonflammability: safety tests of the S-ASSLMB. (a). Voltage profiles of the 4.2V- charged pouch-type cells (SSE vs. conventional electrolyte (1M LiTFSI in EC/DMC = 1/1 (v/v))) at 100°C. Insets show photographs of the cells after the thermal tolerance test. (b) The S-ASSLMB was cut in half horizontally and continued to power an LED lamp without explosion (inset) and even upon exposure to a flame (c, d) Nonflammability test of the coordinated electrolyte (vs. conventional liquid electrolyte (1M LiTFSI in EC/DMC = 1/1 (v/v)) (inset)) and SSE.
53
Figure 2.26. Bipolar configuration of the S-ASSLMBs. (a) Conceptual representation of the bipolar S-ASSLMBs with in-series configuration (1 → 4 cells). (b) charge/discharge voltage profiles of the bipolar S-ASSLMBs (at a charge/discharge current density of 0.1 C/0.1C) as a function of stacked cells. (b) Cross-sectional SEM image (right) of the bipolar S-ASSLMB with 4 cells stacked in-series.
54
Table 2.3 (a) Comparison between the SSE (this study) and previously reported solid-state electrolytes in terms of synthetic conditions, ion conduction characteristics, and electrochemical stability window. (b) Comparison between the S-ASSLMB (this study) and previously reported ASSLMBs in terms of cell components, operating conditions, electrochemical performance, bipolar stacked cells, flexibility, and safety.
a.
Electrolyte classification
Electrolyte composition
Electrolyte synthesis
(temerature, pressure) Transference number Ionic conductivity Electrochemical
stability window Ref.
(°C, MPa) (×10−3 S cm−1) (V)
Single-ion conducting soft
electrolyte
SSE
(Coordinate electrolyte/
Cationic copolymer/
Ti-SiO2@Al2O3)
UV polymerization
(25°C, ambient pressure) 0.91 (25°C) 0.4 (25°C) 6 This
work
Inorganic electrolyte
LGPS Pelletizing
(30Pa, 550°C for 8h) - 12 (27°C) - [15]
LPSCl Coating on a PET film/
Transferring (50 MPa) Unity (-) 1.8 (-) - [17]
LLZTO-Cu3N Pelletizing
(300°C) - 1.1 (-) - [45]
LLZTO–LZO Pelletizing
(1240°C for 30 min) Unity (-) 1.09 (-) [53]
LPSI-xSn SSEs Pelletizing
(300 MPa) - 0.35 (-) - [58]
LLZ:TA Pelletizing/sintering
(150 MPa, 1150°C for 5h) - 2 (-) - [59]
LGPS Pelletizing/sintering
(3 MPa, 950-1000°C for 24h - 5 (-) - [61]
55
LPSCl Pelletizing
(370 MPa) - - - [62]
LPSCl Transferring NW scaffold
(370 MPa) - 0.19 (-) - [64]
LSiPSCl Pelletizing
(-, 240-550°C) Unity (-) 25 (-) - [69]
LLCaZrNbO Pelletizing
(1050°C for 12h) - 0.23 (-) - [70]
Polymer electrolyte
TrIblock copolymer Nitroxide-mediated polymerization
(-,-) 0.85 (-) 0.013 (60°C) 5 [71]
LiBAMB‐PETMP‐
DODT (LPD)
@PVDF SIPE
Electrospinning
(-, 60°C for 12h) 0.92 (50℃) 1.32 (-) 6 [72]
PEO-20% P(SSPSILi- alt-MA)
Solution casting
(-, 80°C) 0.97 (80℃) 0.3 (-) 5 [73]
Composite (inorganic/
polymer) electrolyte
CPMEA-LATP
Pelletizing/sintering (-, 1150°C for 6h) Thermal polymerization
(-, 80°C for 24h)
0.91 (-) 0.1 (65°C) 4.75 [74]
LiMNT-PEC-PTFE- LiFSI/FEC
Mixing/evaporation
(-, 80°C for 12h) 0.89 (-) 0.35(-) 4.6 [75]
LATP-PEO-BPEG
Calcination treatment (-, 900°C for 2h)
Solution casting
0.54 (-) 0.025 (60°C) - [76]
56 (-, 70°C for 12h)
PEO(LiTFSI)-LLZTO Mixing/evaporation
(-, 60°C for 12h) 0.47 (-) 0.016 (30°C) 5.03 [77]
PAN@LAGP-LiTFSI/
PEGDA-LiTFSI
Mixing/evaporation
(-, 120°C for 12h) 0.18 (-) 0.037 (25°C) 5 [78]
57 b.
Electrolyte classification
Electrolyte thickness
Cathode/
Anode
Anode thicknes
s
Cell operatin
g pressure
Cell operating temperatur
e
Charge cut-off voltage
Cycle/
Capacity
Low- temperatur
e performanc
e
Flexibility
Safety (Non- flammability
)
Bipolar configuration
Ref.
(µm) (µm) (MPa) (°C) (V) (#/mAh g−1) (# of stacked
cells) Single-ion
conducting soft electrolyte
50 NCM811/Li 20 0.1 25 3.6 100/185 o o o 4
This wor k
3000-4000 LCO/In - 30 25 3.6 8/120 - - - - [15]
30 NCM/- - 30 60 4.25 1000/- - - [17]
Inorganic electrolyte
300 LCO/Li - - 25 - 300/125.3 - - - - [45]
- LFP/ Li-Al - - 25 4 100/158.9 - - - - [53]
170 LCO/Li - 300 25 4.2 50/123.7 - - - - [58]
500 LCO/LI 200 150 100 - - - [59]
1400 LFP/Li - 3 55 4 100/148 - - - - [61]
600 LCO/Li-in - 770 100 4.3 -/141 - - - - [62]
70 NCM622/LT
O - 370 30 4.3 -/174 - - - - [64]
1000-2000 LCO/LTO - - - 2.6 1000/130 - - - - [69]
58
340 LFP/Li - - 20 4.5 100/153 - - - - [70]
Polymer electrolyte
70-100 LFP/Li - - 60-80 3.8 90/160 - - - - [71]
- LFP/Li - - 40 4 380/128 - o - - [72]
60-80 LFP/Li - - 80 4 100/158.8 - - - - [73]
Composite (inorganic/pol
ymer) electrolyte
500 LFP/Li - - 65 3.8 100/130 - - - - [74]
70 NCM523/Li - - 25 4.3 200/171 - o o - [75]
- NCM622/Li - - - 4.3 360/96.3 - - o - [76]
60 LFP/Li 50 - 25 3.8 200/99.1 - o - - [77]
25 NCM811/Li 40 - - 4.3 100/175 - - - - [78]
59