Pseudocapacitive charge-storage reaction of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>MnO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo form="postfix">∙</mml:mo><mml:mi>n</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>MnO2∙nH2O in several aqueous alkali and alkaline salts solutions, including <mml:math display="inline"><mml:mrow><mml:mi>LiCl</mml:mi></mml:mrow></mml:math>LiCl , <mml:math display="inline"><mml:mrow><mml:mi>NaCl</mml:mi></mml:mrow></mml:math>NaCl , <mml:math display="inline"><mml:mrow><mml:mi>KCl</mml:mi></mml:mrow></mml:math>KCl , <mml:math display="inline"><mml:mrow><mml:mi>CsCl</mml:mi></mml:mrow></mml:math>CsCl , and <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>CaCl</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>CaCl2 , has been studied on fine-grained <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>MnO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo form="postfix">∙</mml:mo><mml:mi>n</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>MnO2∙nH2O thin films and particles which possess the <mml:math display="inline"><mml:mrow><mml:mi>ε</mml:mi><mml:mtext>-</mml:mtext><mml:msub><mml:mi>MnO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>ε-MnO2 -type crystal structure. In situ synchrotron X-ray diffraction analysis shows that charge transfer at Mn sites upon reduction/oxidation of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>MnO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo form="postfix">∙</mml:mo><mml:mi>n</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>MnO2∙nH2O is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charge/discharge cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that <mml:math display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:msup><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math>H3O+ plays the predominant <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:mo></mml:mo><mml:mn>60</mml:mn><mml:mo>%</mml:mo><mml:mo>)</mml:mo></mml:mrow></mml:math>(60%) role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as: <mml:math display="inline"><mml:mrow><mml:mi>Mn</mml:mi><mml:mo>(</mml:mo><mml:mi>IV</mml:mi><mml:mo>)</mml:mo><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo form="postfix">∙</mml:mo><mml:mi>n</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo><mml:mi>δ</mml:mi><mml:msup><mml:mi mathvariant="normal">e</mml:mi><mml:mo>−</mml:mo></mml:msup><mml:mo>+</mml:mo><mml:mi>δ</mml:mi><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>f</mml:mi><mml:mo>)</mml:mo><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:msup><mml:mi mathvariant="normal">O</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>+</mml:mo><mml:mi>δ</mml:mi><mml:mi>f</mml:mi><mml:msup><mml:mi mathvariant="normal">M</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>⇌</mml:mo><mml:mo>(</mml:mo><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:msub><mml:mo>)</mml:mo><mml:mrow><mml:mi>δ</mml:mi><mml:mo>(</mml:mo><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>f</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">M</mml:mi><mml:mrow><mml:mi>δ</mml:mi><mml:mi>f</mml:mi></mml:mrow></mml:msub><mml:mo>[</mml:mo><mml:mi>Mn</mml:mi><mml:mo>(</mml:mo><mml:mi>III</mml:mi><mml:msub><mml:mo>)</mml:mo><mml:mi>δ</mml:mi></mml:msub><mml:mi>Mn</mml:mi><mml:mo>(</mml:mo><mml:mi>IV</mml:mi><mml:msub><mml:mo>)</mml:mo><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub><mml:mo>]</mml:mo><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo form="postfix">∙</mml:mo><mml:mi>n</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>Mn(IV)O2∙nH2O+δe−+δ(1−f)H3O++δfM+⇌(H3O)δ(1−f)Mδf[Mn(III)δMn(IV)1−δ]O2∙nH2O , where <mml:math display="inline"><mml:mrow><mml:msup><mml:mi mathvariant="normal">M</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:mrow></mml:math>M+ is alkali cation.

The solid-electrolyte-interphase (SEI) layers formed on the electrodes of pristine Si and carbon-coated Si (C-Si) particles in Li cells have been studied. The counter electrode is Li, and the electrolyte is <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Li</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>LiPF6 in the mixture of ethylene carbonate and ethyl methyl carbonate. Other than those, such as Li carbonates and fluoride, already known to the SEI of graphite electrode, there were detected significant amounts of SEI species unique to each of the Si electrodes. On the pristine Si electrode, there was concurrence of abundance of C and Si fluorides after long cycles. Coating the Si particles with a graphitized carbon layer has significant effects on the SEI formation. It helps to keep the Si particles remaining integrated after cycling, resulting in a smooth superficial SEI layer. It removes the native oxide layer not only to reduce humidity contamination but also to significantly change the SEI compositions. The SEI of the C–Si electrode shows the absence of Si and C fluorides but the presence of siloxane species. Reaction mechanisms leading to the formation of the fluoride and siloxane species have been proposed, elucidating an important role played by the native Si oxide layer.

The electrochemical mechanism leading to the pseudocapacitance of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>MnFe</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>MnFe2O4 in aqueous electrolyte solutions has been investigated by, in addition to electrochemical characterization, synchrotron X-ray absorption near-edge spectroscopy (XANES) and X-ray diffraction (XRD) analyses using <mml:math display="inline"><mml:mrow><mml:mi>KCl</mml:mi></mml:mrow></mml:math>KCl solution as the model electrolyte. The ferrite crystallites, prepared by a solution method, contain Mn and Fe ions randomly distributed over the tetrahedral and octahedral sites of the spinel structure and exhibit an average capacitance exceeding <mml:math display="inline"><mml:mrow><mml:mn>100</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">F</mml:mi><mml:mo>∕</mml:mo><mml:mi mathvariant="normal">g</mml:mi></mml:mrow></mml:math>100F∕g -ferrite. Based on the data from the in situ XANES and XRD measurements and open-circuit-potential dependence on solution pH, it is concluded that the pseudocapacitance of <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>MnFe</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>MnFe2O4 involves charge transfer at both the Mn- and Fe-ion sites, balanced by insertion/extraction of proton into/from the lattice. In addition, compared with <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>MnO</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo form="postfix">∙</mml:mo><mml:mi>n</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi></mml:mrow></mml:math>MnO2∙nH2O electrode, the ferrite exhibits a far reduced, by <mml:math display="inline"><mml:mrow><mml:mo>∼</mml:mo><mml:mn>90</mml:mn><mml:mo>%</mml:mo></mml:mrow></mml:math>∼90% , lattice expansion upon cycling.