We present the following results: 1) We introduce a doping source for MgB2, liquid SiCl4, which is free of C, to significantly enhance the irreversibility field (Hirr) , the upper critical field (Hc2), and the critical current density (Jc ) with a little reduction in the critical temperature (Tc). 2) Although Si can not be incorporated into the crystal lattice, a significant reduction in the a-axis lattice parameter was found, to the same extent as for carbon doping. 3) Based on the first-principles calculation, it is found that it is reliable to estimate the C concentration just from the reduction in the a-lattice parameter for C-doped MgB2 polycrystalline samples that are prepared at high sintering temperatures, but not for those prepared at low sintering temperatures. Strain effects and magnesium deficiency might be reasons for the a-lattice reduction in non-C or some of the C-added MgB2 samples. 4) The SiCl4-doped MgB2 shows much higher Jc with superior field dependence above 20 K compared to undoped MgB2 and MgB2 doped with various carbon sources. 5) We introduce a parameter, RHH (Hc2 / Hirr), which can clearly reflect the degree of flux-pinning enhancement, providing us with guidance for further enhancing Jc . 6) It was found that spatial variation in the charge-carrier mean free path is responsible for the flux-pinning mechanism in the SiCl4 treated MgB2 with large in-field Jc.

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